Generalized two-dimensional (2D) Fourier transform infrared correlation spectroscopy was used to investigate the effect of the comonomer compositions on the crystallization behavior of two types of biosynthesized random copolymers, poly (hydroxybutyrate-co-hydroxyhexanoate) and poly (hydroxybutyrate-co-hydroxyvalerate). The carbonyl absorption band around 1730cm 1 was sensitive to the degree of crystallinity. 2D correlation analysis demonstrated that the 3-hydroxyhexanoate units preferred to remain in the amorphous phase of the semicrystalline poly(hydroxybutyrate-co-hydroxyhexanoate) copolymer, resulting in decreases in the degree of crystallinity and the rate of the crystallization process. The poly(hydroxybutyrate-co-hydroxyvalerate) copolymer maintained a high degree of crystallinity when the 3-hydroxyvalerate fraction was increased from 0 to 25mol% because of isodimorphism. The crystalline and amorphous absorption bands for the carbonyl bond for this copolymer, therefore, changed simultaneously.