The performance of a two-stage, two-phase, unmixed anaerobic digester of 155 l working volume operating on acid cheese whey and daily manure at various temperatures and hydraulic retention times was investigated The effect of controlling the pH of the methanogonic stage of cheese whey digestion on the biogas production rate and pollution potential reduction was also investigated The digester was designed to act as a liquid-solid separator in order to maximize the microbial mass in the reactor and was operated at three hydraulic retention times (10, 15 and 20 days) and two temperatures (25 and 35 degrees C). It operated as a single-phase reactor under both the cheese whey and dairy manure feeding conditions when the pH was not controlled (similar pH values in the inlet and outlet chambers) and as a two-stage, two-phase reactor when the pH of the whey was controlled in the methanogenic stage (different pH values in the inlet and outlet chambers). The results indicated that production of biogas from cheese whey without pH control is not feasible as the digester experienced acid-phase digestion in both stages. However; controlling the pH of the methanogenic stage increased the biogas production rate and methane yield, as well as the reductions in COD and solids concentrations of the cheese whey by a factor of 2.7-3.0. Although the pH was maintained at 5.7-6.0, the results are comparable to those obtained with dairy manure of a similar solids concentration and a pH of 7+/-0.2. A further increase in the pH (to 7.0+/-0.2) would, therefore, increase the biogas production rate and methane yield from cheese whey. Copyright (C) 1996 Elsevier Science Ltd.