A method of separating 3-nitro-o-toluic acid and 5-nitro-o-toluic acid from a mixture of said isomers, comprising producing salts by adding an aromatic organic base, for example, aniline or pyridine, to the said mixture, separating said salts into solids and a mother liquor, by taking advantage of the difference in solubilities between the said salts to water or a mixed solvent consisting of water and a water-soluble organic compound, for example, methanol, and, further, recovering 3-nitro-o-toluic acid from the mother liquor and 5-nitro-o-toluic acid from the solids.