The energy efficiency of a direct methanol fuel cell (DMFC) is dependent on the cell potential, crossover of methanol and oxygen, and the average number of electrons released per methanol molecule (n(av)) at the anode. The nav value is determined by the product distribution (CO2: formic acid: formaldehyde), and in the absence of methanol loss due to crossover can be determined from the dependence of the cell current on the flow rate of the methanol solution. The effects of crossover have been assessed here by analysis of methanol in the cell exhaust, and a method to correct for crossover is presented. Under mass transport limited conditions, the effect of crossover is shown to be negligible, and accurate nav values are obtained from the flow rate dependence of the current alone. However, analysis of methanol in the cell exhaust is essential for determining nav under mixed kinetic and mass transport control. The exhaust concentration, combined with the flow rate dependence of the current, also provides a measure of efficiency losses due to crossover. (C) 2016 Elsevier Ltd. All rights reserved.