Abstract: The esterification reaction of acetic anhydride and polyphenylene oxide(PPO) was studied by IR and GPC. The results showed that acetic anhydride can react with the OH group of PPO to form ester under heating and catalyzing. Excess acetic anhydride would hydrolyze into acetic acid. The OH group content of PPO can be measured by titration of potassium hydroxide alcohol solution with acetic acid. One kind of PPO was measured by this method and the OH group content was 851.6 g/mol by average. The results were stable.