A detailed structural analysis of the copolyesterification in bulk without any external catalyst at 160 degrees C between o-phthalic anhydride (P), oleic acid (O), and trimethylolpropane (T) with a mol ratio ([-COOH]/[-OH]) = 0.70 has been carried out by high resolution C-13 nuclear magnetic resonance (C-13-NMR) (DMSO-d(6) and CDCl3 solutions), H-1 nuclear magnetic resonance (H-1-NMR) (CDCl3 solution), and by volumetry. The use of CDCl3 as NMR solvent has allow us to identify several signals that have been assigned to trimethylolpropane monoesters with different esterification degrees in the o-phthalate residue. Identically, we have detected signals assignable to monoesters at the chain ends in structures with different chain lengths. These monoesterified structures have been also confirmed by analyzing samples modified by the diazomethane addition. These modified samples have been also used to determine free acid groups as their methoxylic derivatives by H-1-NMR in CDCl3 solution. We have not observed any detectable signs of gelation nor products produced by secondary reactions.