This work studies formic acid crossover in a direct formic acid fuel cell under different operating conditions. Comparison with methanol crossover in a direct methanol fuel cell is included. The effects of cell conditioning, temperature, fuel concentration, and Nafion (R) thickness on the rate of formic acid crossover are examined. The effects of temperature, concentration, and membrane thickness are qualitatively similar to those with methanol, but overall crossover rates are much lower. Under the same operating conditions, it is found that formic acid has a crossover flux rate that is approximately one-sixth that of methanol. Measurement of the CO2 membrane permeation flux is performed to determine its contribution to a bias that it might cause in the quantification of crossover. (C) 2007 Elsevier B.V. All rights reserved.