Conductivity of carboxylic acid membranes can be increased for use in fuel cells when the membrane comprises a Lewis base, which has, in its protonated form, a pKa greater than the pKa of the carboxylic acid groups of the membrane. When the invention is employed in direct methanol fuel cells, water transport and methanol crossover though the membrane are decreased compared to conventional direct methanol fuel cells employing sulfonic acid membranes.