The aim of this study is to investigate the combined effects of Cu2+ and Zn2+ in a synthetic wastewater (WW) on activated sludge process (ASP) over a wide range of mean cell residence times. Experiments were performed in two phases: (i) experiments without heavy metals (HMs), (ii) experiments with HMs using acclimated microorganisms (MOs). In the first phase, biokinetic constants of the synthetic WW inoculated with seed MOs were determined and a linear relationship between substrate removal efficiency and mean cell residence time was found. In the second phase, experiments were conducted with acclimated MOs for processing a WW containing 1.5 mg Cu2+/L and 9 mg Zn2+/L. A variable relationship between substrate removal efficiency and mean cell residence time was obtained. The net result of the presence of Cu2+ and Zn2+ in a synthetic WW was found to be toxic to the MOs in ASP. (C) 1997 Elsevier Science Ltd.