Cellulose in wastewater was converted into H-2 by a mixed culture in batch experiments at 55degreesC with various wastewaters pH (5.5-8.5) and cellulose concentrations (10-40 g l(-1)). At the optimal pH of 6.5, the maximum H-2 yield was 102 ml g(-1) cellulose and the maximum production rate was 287 ml d(-1) for each gram of volatile suspended solids (VSS). Analysis of 16S rDNA sequences showed that the cellulose-degrading mixed culture was composed of microbes closely affiliated to genus Thermoanaerobacterium.