All of 67 samples of sewage, taken at different points from the sewers of the Bonn University hospitals, contained phenolic substances in concentrations ranging from 0.09 mg/l to 5.05 mg/l. About 0.15 mg/l might be caused by human excretion of phenolics. Six samples contained free formaldehyde (2.72-28.38 mg/l), five samples chlorine (0.1 to 1 mg/l). In the main sewer of the hospitals the substances were diluted, but a measurable concentration reached the communities sewage lines. There were no characteristic diurnal changes of the concentrations. Measurements of biological oxygen demand (BOD) in native and artificially prepared sewages using o Sapromat showed, that pure phenol, o-phenylphenol, chlorine and formaldehyde in concentrations as found do not reduce the biologic decomposition. With the exception of chlorine, the substances seem to be integrated into the aerobic microbial decomposition. 3,4 chlorcresol, instead, is able to retard the begin and reduce the amount of decomposition in concentrations, which were found as maximal concentrations for phenolic substances in the sewage samples. Two of 67 samples with the highest concentrations of disinfectants revealed measurable toxity in form of a BOD reduction. The other samples, instead, showed a faster microbiological decomposition than comparable artificial sewage.