In order to clarify the relationships between comonomer-unit compositional distribution and physical properties of bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)], the comonomer-unit composition and its distribution as well as their effects on the physical properties of P(3HB-co-3HHx) have been studied A series of fractions with very different comonomer-unit composition but very narrow distribution were obtained by repeated fractionation and re-fractionation It was found the as-bacterially synthesized P(3HB-co-3HHx) had very broad comonomer-unit compositional distribution The solvent/non-solvent fractionation of P(3HB-co-3HHx) was found to be firstly regulated by the comonomer-unit composition and then by the molecular weight Significant effect of the comonomer-unit compositional distribution on thermal and crystallization behavior as well as crystalline morphology of P(3HB-co-3HHx) was observed It was concluded that as-bacterially synthesized P(3HB-co-3HHx) samples should be considered as polymer blends of component polymers with different 3HHx unit content, whose physical properties considerably depend on the comonomer-unit composition as well as comonomer-unit compositional distribution (C) 2010 Elsevier Ltd All rights reserved