ABSTRACTIn the present study, the effect of chlorpyrifos (CP), its degradation product TCP and N-substituted aromatic compounds (nitrophenol and amino phenol) on methane production from organic solid waste was investigated using co-culture of facultative anaerobe Pseudomonas aeruginosa and anaerobe Methanosarcina mazei. The inhibitors were applied in concentration ranges of 0.5, 1.0, 1.5, 2.5, 3.5 and 5.0 g/L. Results showed that the inhibition coefficient of N-substituted compounds was higher than those of CP and TCP. The maximum inhibition coefficient exhibited by nitrophenol resulted in the lowest methane yield. CP and TCP addition resulted in an average decrease of 27.35% and 52.22% in methane yield, while nitrophenol and aminophenol resulted in a 46.5% and 42.90% reduction in methane yield, respectively. Further analysis revealed that CP and TCP were completely degraded up to a concentration of 1.5 g/L, while nitrophenol and aminophenol were degraded completely up to a concentration of 1 g/L, highlighting molecular weight-dependent inhibition of biodegradation.