Simultaneous measurement of methanol crossover (MCO) at the anode and CO2 crossover (CCO) at the cathode is carried out for the proper evaluation of a membrane-electrode assembly (MEA) in operation of a direct methanol fuel cell (DMFC). The effects of temperature (293-363 K), concentration of methanol in aqueous solution (1-3 mol/dm(3)), current density (0-0.4 A/cm(2)) and gas/liquid flow rate and relative humidity of gas at the cathode on MCO and CCO fluxes are investigated. With an increase in the current density, the MCO flux linearly decreases, while the CCO flux increases to a certain value, and reaches 15% of MCO flux at high current density (0.4 A/cm(2)). The MCO flux is strongly affected by the methanol concentration, temperature and current density, and slightly affected by the gas/liquid flow rate. An empirical equation is proposed for the MCO flux through an MEA in operation of a DMFC under ambient pressure, where liquid methanol and dry oxygen are fed to the anode and cathode, respectively.