The biodegradation of Quercus serrata chips was evaluated by anaerobic digestion under various steam explosion conditions. In continuous experiments, untreated chips (W-0) and chips steam-treated at less than 1.0 MPa (W-1) and 2.0 MPa (W-4) were co-digested with sewage sludge (S-1 and S-2) taken from two different wastewater treatment plants. The apparent methane yield of W-1 and W-4 co-digested with S-1 (thermophilic) was 261 dm(3)/kg VS (volatile solids) and 248 dm(3)/kg VS, respectively. The apparent methane yield of W-4 co-digested with S-2 was 258 dm(3)/kg VS (mesophilic) and 271 dm(3)/kg VS (thermophilic). Methane production was inhibited by W-0 due to components released during hydrolysis. The methane conversion ratio of pretreated chips obtained in batch experiments varied from 40.5% to 53.8% (mesophilic) and from 49.0% to 63.7% (thermophilic). The methane conversion ratio increased with decreasing acid-soluble lignin content in the chips. (C) 2014 Elsevier Ltd. All rights reserved.