The anaerobic treatability of synthetic wastewater containing p-nitrophenol (p-NP) with 80 mg/l concentration was investigated in an anaerobic/aerobic sequential reactor system at organic loadings increasing from 0.3 to 3.16 kg COD/m(3) day. Lab-scale ABR/CSTR reactor system consisted of an anaerobic baffled reactor (ABR) with an effective volume of 28.81 and a completely stirred tank reactor (CSTR) with an effective volume of 91. Organic loading rate was increased step to step from 0.3 to 3.16 kg COD/m(3) day. HRT varied from 1 to 10.38 days corresponding to increasing organic loading rates. A 99% COD and 98% p-NP removal efficiencies were observed for an OLR of 0.3 kg COD/m(3) day (HRT = 10.38 day) in ABR reactor. The COD and p-NP removal efficiencies in ABR reactor decreased from 99% to 71% and from 98% to 90%, respectively, with increasing from 0.3 to 3.16 kg COD/m(3) day of OLR. The overall COD and p-NP removal efficiencies of anaerobic/aerobic sequential reactor system were up to 94% until 2.1 kg COD/m(3) day of OLR. The results related to biogas measurement showed that the total gas and methane gas productions increased while methane percentage decreased with increased OLRs. The daily total gas, methane gas productions and methane percentage were 2220 ml/day, 920 ml/day and 39% at an organic loading rate of 0.3 kg COD/m(3) day, respectively. The aforementioned parameters were found to be 17,696 ml/day, 4320 ml/day and 16% at an organic loading rate of 3.16 kg COD/m(3) day, respectively.