The comonomer-unit compositions and their distribution of as-produced bacterial copolyesters, including poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxy-butyrate-co-3-hydroxypropionate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) are described in this paper. Each copolyester sample can be comonomer-unit compositionally fractionated into several fractions, indicating that the original copolymers are mixtures of copolymers with different comonomer-unit compositions. The effects of comonomer-unit compositional distribution on thermal properties, crystallization, biodegradability and solid-state phase behavior are investigated using comonomer compositionally fractionated copolymers.