Experiments were conducted to investigate the behavior of moving bed biofilm reactor (MBBR) receiving a mixture of toxic (phenolic) wastewater. The study was carried out on laboratory scale using two MBBR reactors fed with synthetic wastewater. The wastewater was prepared by mixing a solution of molasses with a known amount of phenol and nutrients. Two MBBR units were operated simultaneously at different hydraulic retention times (HRT) of 24, 20, 16, 12 and 8 h while phenol concentration was in the range of 200, 400, 620 and 800 mg/l. Throughout the experiments the ratio of phenolic COD concentration to total COD was changed from a ratio of 0.2 to a ratio of 1. The results indicated that the removal efficiency of phenol is affected by the hydraulic retention time and the ratio of phenolic COD concentration to total soluble COD in the reactor feed. At a ratio of 0.6, maximum COD removal efficiency is observed, and this ratio is effective at all HRT's. The effect of hydraulic and toxic shock on the performance of the reactors were examined, and the results proved that the MBBR has good resistance to shock loads and return to steady state condition within two or three cycles of retention time. Microscopic examinations showed that the main bacteria culture attached to carrier elements and biofilms were of filamentous type. (C) 2004 Elsevier Ltd. All rights reserved.