Poly(1,2-dihydroxyethylene-co-vinyl alcohol)s (P(DHE-VA)s) as polymer homologues of methanol/ethanol mixtures were prepared by alkaline hydrolysis of poly(vinylene carbonate-co-vinyl acetate)s. The obtained P(DHE-VA)s were soluble in water reflecting their repeating structures. P(DHE-VA)s were found to show higher thermal stability than poly(vinyl alcohol) (PVA). Membranes composed of P(DHE-VA)s and phosphoric acid exhibited higher ionic conductivity than PVA/phosphoric acid under unhumidified condition, reflecting denser OH groups on the main chains of P(DHE-VA)s.