Laboratory experiments were conducted to study the performance of aerobic/anoxic digestion of mixed primary and waste activated sludge compared to aerobic digestion alone in terms of solids destruction, organic reduction, nutrient removal, filtrate quality and sludge dewaterability. The process performance was examined at different solids residence times (10 and 20 d), temperatures (20, 30 and 40 degrees C) and anoxic cycle length (0, 8, 12 and 16 h). Both modes of operation gave comparable solids destruction results. The volatile suspended solids (VSS) reduction was affected by temperature with the highest VSS reduction reported at 30 degrees C (42.4%), while solid retention time (SRT) did not seem to affect the process as indicated by the percent VSS removal. Biodegradable solids destruction followed a first-order kinetic model. Aerobic/anoxic sludge digestion is advantageous to mere aerobic digestion since it improves sludge dewaterability and filtrate quality. Alternating aerobic/anoxic operation can conserve most of the influent alkalinity and maintain near neutral pH conditions over prolonged periods. Optimum process performance was observed at 20 degrees C with SRT of 10 days and anoxic cycle length of 8 hours.