Batch assays were performed with samples from a silage-fed mesophilic biogas process accumulating acetate to examine if the addition of single trace elements (iron, nickel, cobalt and molybdenum) or a mixture of trace elements could improve the process. The results from the batch assays led to the addition of cobalt to reach a concentration of 0.2 mg L-1. This made it possible to increase the organic loading rate (OLR) above that in a parallel process not receiving any extra cobalt. Problems with low gas production and decreasing pH were overcome by the daily supply of this single element in small amounts. Acetate conversion to methane was improved, which was confirmed by increased specific methanogenic activity (SMA) with acetate as a substrate. As a consequence, an OLR of 7.0 g VS L-1 day(-1) was achieved at a hydraulic retention time (HRT) of 20 days with an equivalent increase in methane production. Without cobalt addition, an OLR of 5.0 g VS L-1 day(-1) with an HRT of 20 days was reached after 70 weeks operation. Copyright (C) 1997 Published by Elsevier Science Ltd.