Biopolyesters poly(3-hydroxybutyrate-co-4-hydroxybutyrate) with an 11 mol % 4HB content [P(3HB-co-11%-4HB)] and a 33 mol % 4HB content [P(3HB-co-33%-4HB)] were blended by a solvent-casting method. The thermal properties were investigated with differential scanning calorimetry. The single glass-transition temperature of the blends revealed that the two components were miscible when the content of P(3HB-co-33%-4HB) was less than 30% or more than 70 wt %. The blends, however, were immiscible when the P(3HB-co-33%-4HB) content was between 30 and 70%. The miscibility of the blends was also confirmed by scanning electron microscopy morphology observation. In the crystallite structure study, X-ray diffraction patterns demonstrated that the crystallites of the blends were mainly from poly(3-hydroxybutyrate) units. With the addition of P(3HB-co-33%-4HB), larger crystallites with lower crystallization degrees were induced. Isothermal crystallization was used to analyze the melting crystallization kinetics. The Avrami exponent was kept around 2; this indicated that the crystallization mode was not affected by the blending. The equilibrium melting temperature decreased from 144 to 140 degrees C for the 80/20 and 70/30 blends P(3HB-co-11%-4HB)/P(3HB-co-33%-4HB). This hinted that the crystallization tendency decreased with a higher P(3HB-co-33%-4HB) content. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3467-3475, 2011