Poly(phenylene oxide)s bearing carboxylic and sulfonic acids were prepared via oxidative polymerization of the corresponding phenol derivatives. Polymerization of 2-(ethoxycarbonyl)6-methylphenol yielded poly [2-(ethoxycarbonyl)-6-methyl- 1,4-phenylene oxide] with a high molecular weight, and its hydrolysis produced poly(2-carboxy-6-methyl-1,4-phenylene oxide) with a series of carboxylic acid contents. Polymerization of 4-bromo-2-(ethoxysulfonyl)-6-methylphenol produced poly(2-methyl-6-sulfo-1,4-phenylene oxide). The acid-functionalized poly(1,4-phenylene oxide)s showed a high thermal stability and proton conductivity.