The Friedel-Crafts self-condensation of 3-aryl-3-chlorophthalides monosubstituted in the phthalide ring has been studied. 3-Aryl-3-chlorophthalides containing Cl, Br, F, and NO2 as substituents were polymerized in solutions of nitrobenzene and dichloroethane in the presence of Lewis acids as a catalyst. The structures of the polymers obtained were established by C-13 NMR spectroscopy. The monomers containing substituents at the 4- and 6-positions of the phthalide ring were found to partially rearrange in the course of the polymer synthesis, giving rise to isomeric units in the polymer. From the effect of reaction conditions on the monomer rearrangement a mechanism has been proposed.