The start-up and operation of an expanded granular sludge bed (EGSB) reactor under psychrophilic (10-12 degrees C) conditions was studied. The reactor was seeded with mesophilic methanogenic granular sludge and fed with a mixture of volatile fatty acids (VFA). Chemical oxygen demand (GOD) removal efficiencies exceeded 90% at imposed organic loading rates up to 12 g COD . l(-1). d(-1) at 10-12 degrees C using influent concentrations ranging from 500 to 800 mg COD . l(-1). The applied hydraulic retention time (HRT) was between 2.5 and 1.6 h and a liquid upflow velocity of 10 m h(-1) was applied. The effect of temperature on the specific VFA conversion rates was assessed using batch activity assays for the seed sludge and the sludge cultivated in the reactor. The optima temperatures for substrate conversion of sludge exposed to long term psychrophilic conditions were similar to those of the original mesophilic inoculum. Both sludges exerted an optimum substrate conversion rate at 35-40 degrees C. The temperature dependence of acetate conversion between 10 degrees C and 40 degrees C could be described by an Arrhenius derived model. Propionate, butyrate and VFA mixtures degrading activities for the same temperature range could be described by a aquare root model. The specific activities of the sludge in the reactor increased in time indicating enrichment of methanogens and acetogens even at low temperature.