The composition and molar masses of equimolar multicomponent copolyesters, obtained from 1,4-butandiol and mixtures of succinic, adipic. sebacic, and terephthalic acids were characterized by C-13 NMR, size exclusion chromatography (SEC), matrix assisted laser desorption ionization-time of flight mass spectroscopy (MALDI-TOF-MS), and viscosimetry. These copolyesters were subjected to partial methanolysis, and the composition and sequence of the oligomers obtained in the methanolysis were determined by the analysis of their fast atom bombardment (FAB-MS) mass spectra. The comparison of the composition data obtained by FAB-MS and C-13 NMR indicates that for copolymers containing sebacic units the methanolysis is a partially selective process, and that oligomers containing sebacic units accumulate in the residue.