Two different terpolyesters were obtained by fermentation of Rhodospirillum rubrum fed either with 4-pentenoic acid alone or with an equimolar mixture of 4-pentenoic and valeric acids. Sequence distributions of these two polyesters were determined by fast atom bombardment mass spectrometry (FAB-MS) of the oligomers prepared by partial methanolysis and separated by HPLC. The distributions in both polymers follow Bernoullian statistics, indicating that both are random terpolyesters.