A study was made of the thermal cyclization (imidization) of functional derivatives of poly-(4-4′-hydroxyphenylene)pyromellitamic acid, namely substituted ammonium salts of the latter, the ethyl and phenyl esters, N-methyl- and N-phenylamides, and also polyisoimide. The functional derivatives were prepared by reactions in the poly(amido acid) chains. The rate constants and activation energies were calculated for imidization. Changes occurring in the deformation-strength properties of the polymers during heat treatment were determined. A relationship between reaction rates and the chemical structure of the derivatives was found as well as evidence to show that the level of the properties of the obtained polyimides is dependent on the nature of the low molecular products formed in the cyclization processes.