To provide thermodynamic data for isonicotinic acid production, the solubilities of isonicotinic acid in water and 4-methylpyridine binary solvent mixture were determined from (287.65 to 361.15) K with the solvent mole fraction composition ranging from 0.00 to 1.00. The experimental data were correlated with the modified Apelblat equation. The calculated results show good agreement with the experimental data.