Sixteen different polyester paint resins have been examined by electrospray ionization mass spectrometry (ESI-MS) in a novel application of the technique for structural analysis of polyesters. Synthetic resins typically contain a range of polymeric structures comprised of various ratios of the monomeric components. When polyesters are analyzed by ESI-MS, singly and doubly charged ions are produced and oligomeric species of the polyesters can be clearly identified. The addition of alkali metal salts to the polyester samples and modification of the solvent system was also found to dramatically enhance the ESI-MS efficiency of polyesters. The ESI mass spectra enabled identification of the range and relative abundance of chemical structures present in each sample. Using this information, average molecular weights, polydispersity, the distribution of acid versus alcohol end groups, overall monomer proportions, average functionality per molecule, and average frequency of branching per molecule have been calculated. The identities of peaks in the ESI-MS spectra have been confirmed by tandem mass spectrometry.