We have investigated the pervaporation separation of methyl t-butyl ether (MTBE)/methanol (MeOH) mixtures using composite membranes prepared by coating a thin film of a polystyrenesulfonate across the surface of a microporous alumina support membrane. Such composite membranes are designated PSS/Al2O3 composites in this paper. The results show that the PSS/Al2O3 composite membranes possess high MeOH transport selectivity, relative to MTBE. MeOH concentrations in the permeate were always greater than 99.5 wt% for all membranes and for all feed solution compositions studied. Membranes in the Mg2+-counter ion form (PSS-Mg) showed higher separation factors than membranes having Na+ as the counter ion (PSS-Na). Extremely high separation factors (25000 to 35000) were observed for the PSS-Mg/Al2O3 composite membrane of 27.5 mol% sulfonate.