PURPOSE: To easily extract dihydrodihydroxyphthalic acid in high efficiency from an aqueous solution containing dihydrodihydroxyphthalic acid by using a specific organic solvent mixture in the presence of an inorganic acid salt.

CONSTITUTION: Dihydrodihydroxyphthalic acid can be separated in high efficiency from an aqueous solution containing dihydrodihydroxyphthalic acid by extracting with an organic solvent selected from (1) propyl alcohol, (2) acetonitrile, (3) a mixture of ethyl alcohol and at least one compound selected from diethyl ether, methyl ethyl ketone and ethyl acetate and (4) a mixture of dioxane and at least one compound selected from butyl alcohol and methyl ethyl ketone in the presence of an inorganic acid salt such as Nail and KCl. Dihydrodihydroxyphthalic acid can easily be produced from phthalic acid by using microorganisms.