Alternating polyesterimides soluble in m-cresol, dimethyl-formamide, and dimethyl sulfoxide have been obtained by the reaction of p,p′-phenolphthalein-bis(trimellitic) dianhydride with primary aromatic diamines. The course of reaction between dianhydride and diamines and the cyclization process of polyesteramidoacids have been studied. One-step synthesis of polyesterimides by polycyclization in m-cresol solution has been developed. The coatings from polyesterimides prepared possess good thermal stability and advantageous mechanical properties.