The solubility data of sodium 4-nitrotoluene-2-sulfonate (NTSNa) and 4-nitrotoluene-2-sulfonic acid (NTS) in (sulfuric acid + water) mixture solvents were measured at temperatures ranging from 285 K to 340 K using a dynamic method. The solute-free mass fraction of sulfuric acid (w(3)(0)) in solvent mixtures ranged from 0 to 0.70. NTS showed the lowest solubility when w(3)(0) is 0.60. After the sulfonation reaction, the reaction solution should be diluted to the concentration 0.60 mass fraction of sulfuric acid to make NTS precipitate as much as possible. Solubility of NTS is much larger than that of NTSNa when w(3)(0) is smaller than 0.20, but no obvious solubility difference can be detected when w(3)(0) is larger than 0.50. This result shows that only when w(3)(0) is smaller than 0.20, the recycle process which was designed by our previous study can be carried out. The solubility data were correlated by the modified Apelblat model, the calculated (solid + liquid) equilibrium curves are in good agreement with the experimental data. (C) 2013 Elsevier B.V. All rights reserved.