Batch tests were used to study the contribution of redox potential to the inhibition of methanogenesis by nitrates. Using cultures containing cysteine solution which lowered the redox potential to about -300 mV, it was found that methane production started only after the total reduction of all oxides of nitrogen. However, it was observed that cysteine enhanced methanogenesis for cultures not containing nitrate. These cultures produced more methane than those without cysteine. Furthermore, cysteine was also found to enhance denitrification. Denitrification rates were 7 and 3.5 mg NO3-N gVSS-1 h-1 for cysteine and non cysteine-containing cultures respectively. This study shows that to have simultaneous denitrification and methanogenesis processes in a continuous single reactor would require a reactor system with zones of different nitrate concentrations, so that denitrifying and methanogenic populations could coexist and flourish.