Three 51 completely stirred tank reactors (CSTRs) were used to examine the thermophilic anaerobic digestion of waste activated sludge. The objective of the study was to determine the most appropriate hydraulic retention time for the use of such a system as the first stage of a thermophilic/mesophilic dual digestion process. The hydraulic retention times used ranged from 48 hours to 8 hours. The results showed that, with hydraulic retention times in the range 18-48 hours, the digesters functioned as single-stage units with little accumulation of volatile fatty acids (VFAs). At lower hydraulic retention times, acids did accumulate. Gas production showed a similar pattern except that the critical retention time was 14 hours. The results show that a thermophilic CSTR can be used as the first stage of a dual digestion process with low hydraulic retention times and, therefore, with a compact design.