Polyhydroxyalkanoate (PHA) copolyesters were synthesized by Cupriavidus necator cells in continuous feeding of cosubstrates. During the PHA accumulation phase, the composition of 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), and 4-hydroxyvalerate (4HV) of the copolyesters changed with time, resulting in a change in their miscibility. The as-produced PHA finally became a miscible blend of copolymers with a broad chemical composition distribution. The good miscibility and low crystallinity of the natural P(3HB-co-3HV-co-4HV) blend lead to a remarkable increase in ductility and elongation at break. It indicates that the material properties of copolyesters can be tailored via feeding control of cosubstrates. It was also found that the fractions of natural PHA blend exhibited distinctive thermal behavior and the overall behavior of the as-produced PHA blend was primarily dependent on a fraction of high 3HB content. The material properties of a PHA blend are therefore not determined by its overall chemical composition but more likely by the combined effect of individual copolyesters or fractions. Moreover, the degree of X-ray crystallinity of random P(3HB-co-3HV-co-4HV) blend declined significantly with the increase of 3HV and 4HV content, in contrast to the high crystallinity of well-known P(3HB-co-3HV) copolyesters. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013