During high-temperature condensation of six-membered naphthalene anhydrides with aromatic primary amines, at first, naphthalisoimides were formed, having cis or trans structures in dependence on absence or presence of acidic catalysts. Only the trans naphthalisoimides isomerized to the naphthalimides in the presence of basic catalysts during heating or UV irradiation. The mechanism of napthalisoimides formation and their rearrangement to naphthalimides has been proposed.