The performance of an engineered membrane assembly in direct methanol fuel cells (DMFC) with an open- cathode configuration is evaluated in terms of experimentally measurable performance metrics that allow the quantitative characterization of the water balance in the stack as well as the extent of methanol crossover experienced at various operating conditions. A review of the definition and measurement of a water-balance parameter, and its associated water-vapor mass transfer parameter, is presented to illustrate the methodology. In addition, the methanol crossover effect is quantified indirectly through the measurement of the amount of carbon-dioxide gas present in the cathode exit stream. Details of the instrumentation and experimental configuration used to make the needed measurements are provided, including the adoption of a cathode plenum and a bypass stream that enables the capture and on-line analysis of gas samples from the cathode exit stream. Studies are conducted in a fuel cell system that includes membrane assemblies fitted with a hydrophobic liquid barrier layer as well as a liquid diffusion layer featuring flow channels that promote the back flow of water through the membrane towards the anode side. The experimental results show that the proposed parameters show a linear dependence on the current density under isothermal stack conditions. On the other hand, at a given operating current density the parameters change in a nonlinear fashion with respect to stack temperature variations. The effect of the bulk methanol concentration in the fuel stream is also shown to have a nonlinear effect on the methanol crossover at constant current density operating points. The results lend support to the claim that the proposed metrics provide a venue for material designers to test and re-engineer specific layers of the DMFC assembly, with the goal of attaining optimal fuel-cell performance.