The solubilities of p-toluenesulfonic acid monohydrate (PTSA center dot H2O) and sodium p-toluenesulfonate (NaPTS) in aqueous sulfuric acid solutions were investigated at a temperature range from 278.45 to 342.95 K. The experimental results showed that the solubilities of PTSA center dot H2O and NaPTS in aqueous sulfuric acid solutions increased with temperature from 278.45 to 342.95 K and decreased with sulfuric acid concentrations from 0 to 0.7. The experimental data were correlated with the modified Apelblat equation. The calculated results showed good agreement with the experimental data. A new strategy which sodium sulfate was used as a starting material in the preparation of NaPTS was profiled according to the solubility difference between PTSA center dot H2O and NaPTS. In the continuous preparation of NaPTS, the favorable mole ratio (PTSA center dot H2O to sodium sulfate) is chosen as 4.0, and the optimum operation conditions of the recycle process were determined, which improved the present process of 4-methylphenol production.