Methanogenic cultures were acclimatized to operate at low pHs and the efficiency of methane production at pH 5.0, 4.5 and 4.0 was 67, 37 and 34% respectively, compared to biomethanation at pH 7.0 (100% yield) with 55-65% methane in the biogas composition. Above pH 5.0, the efficiency of methane production was more than 75%. The data for most of the experiments are based on the observations carried out for more than 60 and 100 days. The control experiments of direct pH shock of 4.5 and 4.0 did not show any gas production and even at pH 5.5 production of methane was marginal.