The effect of low operating temperature and pollutant concentration on the performance of five anaerobic hybrid reactors was investigated. Stable and efficient long- term (. 400 days) treatment of a cold ( 6 - 13 degrees C), volatile fatty acid ( VFA)- based, wastewater was achieved at applied organic loading rates ( OLRs) of 5 kg chemical oxygen demand ( COD) m(-3) d(-1) with COD removal efficiencies c. 84% at 6 degrees C ( sludge loading rate ( SLR) 1.04 - 1.46 kg COD kg [ VSS](-1) d(-1)). VFA- based wastewaters, containing up to 14 g pentachlorophenol ( PCP) m(-3) d(-1) or 155 g toluene m(-3) d(-1) were successfully treated at applied OLRs of 5 - 7 kg COD m(-3) d(-1). Despite transient declines in reactor performance in response to increasing toxicant loading rates, stable operation ( COD removal efficiencies. 90%) and satisfactory toxicant removal efficiencies (. 88%) were demonstrated by the systems.