Nafion-montmorillonite nanocomposite membranes are reported to be effective for reducing fuel crossover when used as the electrolyte in fuel cells. The synthesis of a membrane involves several adjustable parameters. Using the statistical design-of-experiment method, four of the most critical parameters were studied: choice of mixer, clay percentage, curing temperature, and curing time. Fuel crossover and conductivity were measured for each of the membranes, to assess how the parameters influence the properties of the membranes. Curing time is critical for hydrogen crossover reduction. Four different two-factor interactions are critical for methanol crossover reduction: clay percentage and curing temperature; type of mixer and curing temperature; type of mixer and clay percentage; and curing temperature and time. Clay percentage and curing temperature individually influence in-plane conductivity, as does the interaction between the type of mixer and clay percentage. The factors that influence through-plane conductivity are the type of mixer and clay percentage individually and the interaction between clay percentage and curing temperature. More membranes were made by using the determined best process and varying the type of clay and the clay percentage. The same crossover and conductivity studies were performed, and the results are reported.