Poly(2,6-dimethylphenylene oxide) (PPO) was sulfonated to varying degrees using different sulfonating agents. Physical properties such as solubility, density, and thermal properties were studied for both PPO and sulfonated PPO (SPPO) with different degrees of sulfonation. Density and T-g value were increased after sulfonation. The solubility in dipolar aprotic solvents was improved and was dependent on the degree of sulfonation, and the thermal stability of PPO was slightly decreased after sulfonation.