A two-phase anaerobic process to produce hydrogen and methane from swine manure was investigated, using pretreated sludge with heat, acid and alkali treatment as inoculum. The relative order of pretreatment methods of H-2 productivity effectiveness and CH4 productivity effectiveness produced by the residua of the first phase was heat treatment > alkali treatment > acid treatment. When the inoculum sludge was heat-treated at 80 degrees C for 30 min, the H-2 and CH4 production rate was the highest of 36.6, 201.7 ml (g TS)(added)(-1). There were significant correlations between biogas production and accumulation of acetic acid and butyric acids. When propionic acid and total VFA concentrations reached about 2850 mg L-1 and 10.0 g L-1, respectively, the average H-2 production rate and H-2 content decreased from 7.6 ml d(-1)(g VS)(added)(-1) and 55.3% to 1.4 ml d(-1)(g VS)(added)(-1) and 43.2%, respectively. The activity of methanogenic bacteria was inhibited to a significant extent when the total VFA concentration was above 10.0 g L-1, but this inhibitory effect weakened when the VFA concentration fell to 6200-8500 mg L-1. Correspondingly, average CH4 production rate increased from 4.0 ml d(-1)(g TS)(added)(-1) to 12.5 ml d(-1)(g TS)(added)(-1) Propionic acid was degraded rapidly only when acetic and butyric acid concentrations dropped to 2500 mg L-1 and 1000 mg L-1, respectively. (C) 2009 Elsevier Ltd. All rights reserved.