The solubility of o-phthalic acid in methanol + water and methanol + butyl acetate was measured at temperatures ranging from (295.87 to 359.75) K using a laser-monitoring observation technique at atmospheric pressure. The experimental data were correlated with the Apelblat equation. The overall sigma of 159 data points was 0.70%. The apparent molar enthalpies of o-phthalic acid in the two systems have been determined and provide a heat transmission basis for the design of a crystallizer.