The gas transport properties of surface-treated poly(3-(2-acetoxyethyl)thiophene) (P3AcET) membranes are reported. Base or acid catalyzed hydrolysis of the ester group of P3AcET yields poly(3-(2-hydroxyethyl)thiophene) (P3HET), a highly permselective conducting polymer. Permeability coefficients were determined for as-cast P3AcET membranes (P-CO2 = 1.42, P-O2 = 0.24, P-N2 = 0.05, P-CH4 = 0.08 Barrers) and selectivity values were calculated (O-2/N-2 = 5.1, CO2/CH4 = 18.5). Formation of a chin selective P3HET surface layer resulted in an overall decrease in permeance (P11) accompanied by a dramatic increase in selectivity for both the base-treated (O-2/N-2 = 12.9, CO2/CH4 = 20.0) and acid-treated (O-2/N-2 = 11.7, CO2/CH4 = 45.0) composites. Microscopic (AFM, SEM) and spectroscopic (ATR-FTIR) characterization of the thin film composites confirmed the surface specificity of the hydrolysis reaction.