PURPOSE: To obtain 4-chloronaphthalic acid useful as an intermediate for optical whitening agent, etc. or its anhydride advantageously, by reacting naphthalic acid which is made into an aqueous solution with a polybasic inorganic acid salt with chlorine, with suppressing side reactions and abbreviating a neutralization process.

CONSTITUTION: Naphthalic acid is dissolved in an aqueous solution of a polybasic inorganic acid salt such as a borate, phosphate, silicate, or vanadate, etc. and reacted with chlorine in a pH of prefeably 6W9 and at -5W25°C, to give 4-chloronaphthalic acid or its anhydride. By this process, side reactions and oxidative reaction caused by the partial change in pH are prevented by the buffer action of the salt, so that the yield and quality are improved. Operation efficiency is also improved by the abbreviation of the neutralization process, the chlorination reaction is promoted smoothly by the catalytic effect of the salt, and satisfactory results can be obtained by nearly theoretical amounts of the raw materials in wide ranges of pH and temperature.