A method and apparatus for purifying carboxylic acids is disclosed. A mixture containing crude carboxylic acid is contacted with a selective crystallization solvent to form a slurry of a salt complex of the carboxylic acid and the selective crystallization solvent. The salt complex is recovered and optionally processed to recover the free carboxylic acid. The method and apparatus of the invention is particularly suitable for purifying aromatic dicarboxylic acids such as terephthalic acid. The present invention also reduces contamination by carboxybenaldehyde isomers in crude phthalic acids by oxidizing the carboxybenzaldehyde to the corresponding phthalic acid.