This paper investigates the possibility of attaining sulphonated poly(phenylene oxide) (SPPO) with a relatively higher sulfonation degree. To achieve this aim, the approach we adopt is to improve the solubility of the final product in the mixed solvent so that the sulfonation may take place between the bulk solutions and PPO powders even at higher sulfonation degree. It is shown that the addition of a proper amount of dimethyl formide (DMF) to the conventional PPO-chloroform system can actually enhance the sulfonation effect. The solvent composition is then correlated with the sulfonation degree based on the solubility parameters. It is interesting to find that solubility parameters between the mixed solvent and the precipitated products keep an approximately unchanged value at about 4.9, which is just equal to that when pure chloroform is used, though the solubility parameters of both solvents increase with the content of DMF in solution. This may be the main reason why the addition of DMF can reduce the precipitation and improve the ion exchange capacity (IEC) of SPPO polymer.