Detailed in-depth knowledge of acidogenesis is important for the development of robust and optimised anaerobic digestion processes, and also to selectively produce certain acids or other by-products such as hydrogen during the conversions. Although the complete anaerobic digestion process has been studied for a long time, the detailed understanding of the acidogenic phase is surprisingly limited. In addition to the diversity and complexity of the conversion processes, the highly dynamic nature of the microbial transformations makes it difficult to investigate them properly. Application of on-line monitoring systems would therefore be beneficial in gaining a more detailed and better knowledge of these processes. This thesis focuses on the intensive study of the dynamic acidogenic process to contribute further information for describing these processes in more details, which may lead to improved practical optimisation and control developments.The CSTR parent reactor established to study the high rate acidogenic processes was operating stably for about 18 months with an organic loading rate (OLR) of 16 kg COD m-3 d-1 and a hydraulic retention time (HRT) of 8 hours. The performance data from the steady-state long-term operation could be balanced for both the COD and carbon content of all incoming substrates (only glucose) and outgoing products with a low percentage of error. This indicates that all the relevant carbon or COD-containing compounds were accurately measured in the system. The results could therefore be used as control data for other studies under different operating conditions including variation of the OLR and /or the HRT. Both the product composition with butyric and acetic acids as main products and the microbial population containing a large fraction of CIostridium-related biomass is consistent with the typically observed butyric acid type fermentation found under such operating conditions as reported in the literature.The suitability and benefits of the application of the titration and off-gas analysis (TOGA) sensor for studies of the anaerobic acidogenic process were investigated under steady-state and dynamic conditions. Complete mass balances could be accurately calculated from the data and even rapid, dynamic changes could be followed well. The closure of the COD and carbon balances to within a few percent under general operating conditions is particularly important as it demonstrates that all of the relevant products are being detected with the analysis methods applied in these studies. Distributions of acidogenic products were also calculated to explore the effects of the transfer of the culture from the parent reactor to the test reactor and the product variation in relation to the changed test conditions in the TOGA reactor. It was clear that the culture transfer from the parent reactor to the test reactor had only a small impact on the process characteristics. These studies show that the TOGA sensor has a major potential as a reliable tool for anaerobic and particular acidogenesis studies under both steady-state and dynamic conditions.The study of dynamic acidogenic process with batch feeding experiments represented significant changes of the reactor performance and biomass behaviour, which was particularly obvious from the changes in the product distribution, rate of substrate consumption and production rate of metabolites. Frequent and accurate measurements of the changing compounds in both the liquid and gas phase could be achieved in the short-term investigations based to the application of the TOGA sensor and frequent sampling. In some cases, there appears to be a temporary production of unknown (and unmeasured) products occurring, which may be identified from the significant discrepancy in the COD and carbon mass balances during some batch test periods. The COD/carbon ratio of the missing fraction indicated that it might be a quite oxidized product, possibly even carbon monoxide (CO). Furthermore, a significant increase of lactic acid production in the batch tests was observed, particularly when the biomass was adapted to a higher organic loading rate (OLR) and shorter hydraulic retention time (HRT). Interestingly, a higher concentration of lactic acid also existed in the continuous test period with the biomass from the parent reactor operated under high OLR and short HRT. This confirms previous reports that lactic acid could be used as a possible indicator for an overloading situation in the acidogenic process.The dynamic acidogenic process in the continuous feeding system was clearly affected by the operational conditions. The organic substrate overload tests showed that the process was able to operate quite stably with increasing metabolic production and higher gas flow rates. The hydraulic overloading was more critical and could lead to a biomass wash-out at least in one case with an HRT of 3.7 hours. While in this case the dilution rate clearly exceeded the specific growth rate as expected, two other experiments with reduced HRT seem to operate stably despite the calculated specific growth rates being lower than the dilution rates, which could not be explained from the results.The accumulation of the main metabolic products, n-butyric acid and acetic acid, appears to inhibit the glucose consumption and metabolite production rates, whilst high concentrations of lactic acid in the system may not have any significant impact. This would constitute a direct product inhibition effect which is not unexpected given the high levels of these acids present in these tests. The results furthermore demonstrate that there was no significant evidence of any further degradation of lactic acid or n-butyric acid in this acidogenic process. It could therefore be concluded that these acids are only end products of this high-rate process. They do not appear to be involved as intermediates as they would otherwise be expected to be degraded quite rapidly, as was observed in other studies reported in the literature. The results show clearly that this specialist culture was not able to degrade these common or temporarily occurring metabolic products. The rapid degradation of lactic acid, as well as the absence of large amounts of n-butyric acid in the previous studies might suggest that a different and likely more diverse population of microorganisms was present in those systems.From the comparison of the stoichiometry and kinetics across all of the experimental conditions, it seems that there is no clear trend visible in relation to the tested operating parameters. Nevertheless, there are major effects obvious from the data relating to the various disturbances tested in these studies. It can be summarised that the measured stoichiometric and kinetic information are only applicable to the specific culture and conditions. However, it might be possible to determine some reasonably accurate averaged parameters which might be applicable to a range of conditions.