We investigated the thermal decomposition behavior of three groups of polyesterimides that had been synthesized from different compositions of monomers that were added in different. We characterized these polymers with thermogravimetric analysis (TGA) and calculated the apparent activation energy (E-n) associated with the thermal decomposition process by the Ozawa method. The results showed that the E, of the polyesterimides was correlated with the length of the methylene spacer and the content of the 4,4'-dihydroxybenzophenone monomer. The polyesterimide with four methylene spacers in the main chain had a higher E, than that with six methylene spacers. The polyesterimide with a higher 4,4'-dihydroxybenzophenone content provided better thermal stability. The E. of the polyesterimides also depended on the sequence in which the monomers were added during the copolycondensation process. The E, of these polyesterimides followed the order: p-hydroxybenzoic acid added first > p-hydroxybenzoic acid mixed 4,4'dihydroxybenzophenone adding > 4,4'-dihydroxybenzophenone added first. (c) 2005 Wiley Periodicals, Inc.