Process for the preparation of naphthalene-1,8-dicarboximide which is optionally substituted in the naphthalene ring, characterized in that naphthalene-1,8-dicarboxylic anhydride which is optionally substituted in the naphthalene ring is reacted with an ammonium salt in an aqueous medium at a pH near to the neutral point, the reaction mixture is then acidified to a pH of 1 to 4 and the pH is subsequently adjusted to a value near to the neutral point.