Asymmetric polyelectrolyte complex (PEC) membranes composed of chitosan membrane and absorbed poly(acrylic acid) (PAA) were constructed. Absorption and therefore PEC formation were performed by bringing a chitosan membrane into contact with a PAA aqueous solution. The mean molecular weight (MW) of PAA employed was 5,000, 25,000 or 250,000. Absorption of PAA and asymmetry of PEC membranes were confirmed and evaluated by Fourier transform infrared (FTIR) spectra and electron spectroscopy for chemical analysis (ESCA). The absorption quantity of PAA decreased while the water selectivity of the PEC membrane increased with an increase in the MW of constituent PAA. In a pervaporation experiment, the water selectivity of the membrane was so high that no ethanol was detected by gas chromatography (GC). Such selectivity was attained by just a small quantity of PEC formation at the surface of the chitosan membrane.