The aim of this study was to investigate the effect of the ratio between COD and biomass (SO/Xo) on inert COD fraction in pharmaceutical wastewaters under aerobic and anaerobic conditions. In this concept, wastewater diluted to 2000 mg.L-1 with the same initial COD and biomass concentration of 50 mg.L-1 were treated in batch reactors to determine the inert COD fraction. The pharmaceutical wastewater consists of a lot of volatile organic compounds, and high concentrations of COD may inhibit particularly the aerobic systems. The aerobic and anaerobic batch runs were operated for 37 days and 86 days, respectively . The results of the aerobic inert COD experiment indicated that the biological removal was only 19%, and 81% of the initial COD level has been removed by air stripping. The anaerobic experiments were carried out at five different S-0/X-0 ratios and an efficiency of 88 % could be provided. It was seen that increasing S-0/X-0 ratios did not change the soluble inert COD (SI). Some unexpected peaks could be observed in raw wastewater, possibly originating from additive effects of particular wastewater fractions increasing COD by degradation or destroyed cell material. Additionally, the concentrations of the volatile fatty acids (VFAs) and their behavior in the inert systems were analyzed and discussed.