The influence of carbon dioxide (CO2) in the headspace gas on the specific methane (CH4) production of blank samples with just inoculum during Biochemical Methane Potential (BMP) tests was studied. The headspace of the bottles had been flushed with 15 different ratios of CO2 and N2 prior to incubation, while they were treated otherwise identically. The results revealed that the CH4 yield increased linearly with higher ratio of CO2 in the flush gas reaching a 30% higher yield at pure CO2 relative to pure N2 headspace conditions. However, a slightly distinct lag is noticeable during the initial phase of the degradation process at high ratios of CO2, hypothesizing a reversible disturbance of the biocenosis. Further experiments and analyses need to be performed to elucidate the underlying mechanisms.