The microbial population in an activated-sludge reactor can grow and function within a pH range around neutrality. Upset of the activated-sludge system can occur when the reactor pH falls outside this narrow range. An alkaline-tolerating culture was used in this study for augmentation of a laboratory-scale activated-sludge system that suffered abnormal pH. During pulse shocks of pH 12 in the influent, treatment efficiencies of both an augmented reactor and an unaugmented control were found to decline when the pH value of the reactor went above 9. During a prolonged period of elevated influent pH, the augmented reactor had some chance of performing slightly better than the unaugmented control. Both reactors failed because the activated sludge was washed out after about one mean cell residence time. (C) 1997 Elsevier Science Ltd.