Lab-scale continuous flow activated sludge systems that were acclimated to 2,4-dichlorphenoxyacetic acid (2,4-D) under sole 2,4-D influent and without sludge wastage, were able to maintain successful 2,4-D treatment when both 2,4-D and a biogenic substrate were fed and the systems operated with finite mean cell residence times (theta(c)). When the systems were fed dual 2,4-D and biogenic substrates and operated with finite theta(c) from the start, treatment of 2,4-D fluctuated noticeably long after acclimation. At the reintroduction of 2,4-D after its absence from the influent for a period of time (2,4-D shock), the systems under both the sole and dual substrate conditions suffered similar treatment losses; the extent of treatment losses was related to the length of 2,4-D absence time. When shocked, systems with sole 2,4-D influent had a slight advantage over dual substrates by showing a faster recovery from shocks with the help of re-acclimation. (C) 2006 Elsevier Ltd. All rights reserved.