The importance of cobalt on the anaerobic conversion of methanol under thermophilic conditions was studied in three parallel lab-scale UASB-reactors and in cobalt-limited enriched cultures. Reactors R1, R2, and R3 were fed with methanol in a bicarbonate-buffered medium, supplied with iron and macronutrients: in R1 all metals were supplied (control), R2 was cobalt deprived, and in R3 all metals were deprived. In the 136 days of continuous experiment, a drop in performance was observed over the last 30 days. Particularly in R3, both methanol removal and methane formation dropped by 7.1 % and 13.7%, respectively, compared to the control reactor, R1. When the medium was cobalt-deprived, acetate was not produced and, as a consequence, the enriched consortium lost its capacity to degrade acetate, indicating that the acetotrophic microorganisms were washed out. The addition of 0.5 muM of cobalt to a cobalt-deprived enrichment culture led to acetate accumulation. The results obtained in this study indicate that the mixed consortium requires a proper amount of cobalt, and its addition to a concentration of 0.1 muM leads to the highest methanol conversion rate, with methane as the sole end product from methanol. (C) 2004 Wiley Periodicals, Inc.