Alkaline membranes based on quaternized poly(arylene ether sulfone) (QPAES) were characterized in relation to their water and methanol uptake, methanol permeability, electrical conductivity, and mechanical properties. The performance of QPAES as electrolyte in alkaline direct methanol fuel cells was studied using a free-breathing single fuel cell at room temperature. Methanol uptake by QPAES membranes is lower than water, while their methanol permeability, determined in the temperature range from 30 °C to 75 °C, was much lower than for Nafion membranes. Young modulus of QPAES membranes decrease with the degree of alkalization of the membrane, although mechanical properties are still satisfactory for fuel cell applications for membrane alkalized with 2 M KOH, which additionally exhibit optimal hydroxide conductivity. Although the specific conductivity of QPAES membranes was lower than that reported for Nafion, its methanol selectivity (conductivity/methanol permeability ratio), is much higher than that reported for Nafion 117, and a commercial amminated polysulfone. In view of these results, QPAES membranes are expected to exhibit promising performance as an electrolyte in alkaline direct methanol fuel cells.