The effects of a transient external voltage application on the bioanode performance of microbial fuel cells (MFCs) inoculated with mixed cultures were investigated. Different positive and negative external voltages were applied to a set of bioanodes. The MFCs under +1, -1, and -5 V voltage applications achieved higher current densities than the control during the start-up period. The MFC exposed to a voltage of +1 V had the highest maximum power density of 73.5 mW m(-2) after a 96-h operation. However, the +5 and +10V voltage applications delayed or even deteriorated the MFC start-up. The -10 V voltage application initially induced a higher power output, but later had a detrimental effect on the MFC performance. The negative voltage application was proven to enhance the catalytic activity of the bioanode, and found to be partially responsible for the improved MFC bioanode performance. (C) 2010 Elsevier Ltd. All rights reserved.