The effect of the trace metal cobalt on the conversion of methanol in an upflow anaerobic sludge bed (UASB) reactor was investigated by studying the effect of cobalt deprivation from the influent on the reactor efficiency and the sludge characteristics. A UASB reactor (30 C; pH 7) was operated for 261 days at a 12-h hydraulic retention time (HRT). The loading rate was increased stepwise from 2.6 g chemical oxygen demand (COD)?L reactor-1?d-1 to 7.8 g COD?L reactor-1?d-1. Cobalt deprivation had a strong impact on the methanogenic activity of the sludge. In batch tests, the methanogenic activity of the sludge with methanol as the substrate increased 5.3 (day 28) and 2.1 (day 257) times by addition of 840 nM of cobalt. The sludge had an apparent Km for cobalt of 948 nM after 28 days of operation and 442 nM at the end of the run. Cobalt deprivation during 54 days of operation led to a methanol conversion efficiency of only 55ÐContinuous addition of cobalt (330 nM) for 33 days improved the methanol removal efficiency to 100ÐIn this period of cobalt dosing, the cobalt concentration in the sludge increased 2.7 times up to 32 g?g TSS-1. Upon omission of the cobalt addition, cobalt washed-out at a stable rate of 0.1 g?g VSS-1?d-1. At the end of the run, the cobalt concentration of the sludge was similar to that of the seed sludge.