Sequence distribution and chemical structure of mass-selected macromolecules of poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, biopolyester macroinitiator obtained by partial alkaline depolymerization of natural PHBV containing 5 mol % of HV units was determined by electrospray ionization multistage mass spectrometry (ESI-MSn). On the basis of the ESI-MSn studies of PHBV individual;oligomers, the microstructure of this bacterial copolyester was assessed, starting from dimer up to the oligomer containing 22 repeat units, and the results obtained were compared with those described previously for other "soft" ionization MS techniques.