The CO2 transport in the proton form Nafion membranes (E.W. 1100) under various hydration and temperature conditions were studied by using an IR based CO2 detector to monitor both the transient and steady state CO2 permeate flux through the membranes. A time-lag method was used to extract the CO2 diffusivity and solubility from the CO2 permeation-time curve. It was found that the CO2 diffusivity in a dry membrane is very low, but it increases rapidly with the membrane-hydration level. A high CO2 solubility was observed in dry membranes as compared to those in humidified membranes, for which the CO2 solubility remains nearly invariant to the membrane hydration level at a given temperature. By comparing the measured data with those calculated for the CO2 transport solely in the water channels, it is concluded that the CO2 transport in Nafion membranes is dominated by its transport in the water channels. CO2 and methanol crossovers in a direct methanol fuel cell (DMFC) configuration were also examined. By operating the DMFC configuration in methanol electrolysis mode, one can measure the CO2 crossover rate, which can be used to correct the methanol crossover measurements from measuring CO2 flux in the DMFC cathode effluent stream, and to derive the local CO2 partial pressure at the DMFC anode catalyst layer. (C) 2015 The Electrochemical Society. All rights reserved.