The influence of operational conditions (including the specific substrate concn in the feed, the hydraulic retention time and wastewater composition) on the rate of immobilization of acetogenic and methanogenic bacteria in an anaerobic polyurethane carrier reactor (PCR) treating synthetic Volatile Fatty Acids (VFA) based wastewater was investigated. The performance of continuous and fed batch culture reactor systems had a roughly similar start up period and biogas production rate. Preliminary results indicated that these systems started up within 8 d of operation. A max. concn of 151 mM acetate and 126 mM propionate was applied in the reactors treating VFA based wastewaters with a higher proportion of acetate and propionate, respectively. In all reactors a max. concn of 45.4 mM butyrate was applied. At max. acetate concn no toxic effect on acetogenic and methanogenic biomass activity were observed. The max. propionate concn showed strong inhibitory effect on biomass activity. At the max. feed concn the VFA conversion, methane production rate and methane content decreased. Biomass activiy tests showed that the reactors fed with a higher acetate concn showed higher acetogenic and methanogenic activity than the reactor treating wastewater with higher propionate concn. The influence of operational conditions on biomass washout were also investigated.