the process of this invention for preparing 2 , 6 - naphthalene - dicarboxylic acid comprises a reaction step wherein 2 , 6 - naphthalene - dicarboxylic acid potassium salts consisting of 2 , 6 - naphthalene - dicarboxylic acid dipotassium salt and / or 2 , 6 - naphthalene - dicarboxylic acid monopotassium salt are allowed to react with benzene - carboxylic acids in the presence of water to yield 2 , 6 - naphthalene - dicarboxylic acid and benzene - carboxylic acid potassium salts and a separation step wherein the crystallized 2 , 6 - naphthalene - dicarboxylic acid is separated from the benzene - carboxylic acid potassium salts dissolved in the aqueous solution and provides 2 , 6 - naphthalene - dicarboxylic acid , useful as raw material for polymers , at low cost on a commerical scale with recycle of potassium .