In this study, we used the electrochemical impedance spectra to evaluate the anode performance of direct formic acid fuel cell (DFAFC), and how its anode charge transfer resistance (R-anode,R-ct) and electrolyte resistance (R-ele) are affected by various cell operating parameters. The parameters investigated in this study include the anode overpotentials, cell operation times, formic acid feed concentrations and cell temperatures. The anode impedance spectra demonstrated that the R-anode,R-ct and R-ele are low for the DFAFC using 5 M formic acid feed concentration, which leads to its high power density output of 250 MW cm(-2) at 0.35 V and 30 degrees C. The high performance of the DFAFC demonstrates that it has a great potential for portable power applications. The R-anode,R-ct increases gradually as either the cell operation time increases or the formic acid feed concentration is raised from 10 to 15 M, which leads to a deactivation of the anode electrode, resulting in reduction of overall cell performance. However, these deactivation processes are reversible and the cell performance can be easily reactivated. (C) 2007 Elsevier B.V. All rights reserved.