Composition-, temperature-, and polarization-dependent low-frequency Raman and terahertz (far-infrared; FIR) spectra were measured for poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(HB-co-HV)) with PHB-type crystal structure and PHV-type crystal structure to investigate their intermolecular interactions. Band assignments were attempted by comparing the low-frequency Raman and terahertz (FIR) spectra of P(HB-co-HV) with the varying HV content with the corresponding spectra of PHB and by observing spectral variations. The spectra of P(HB-co-HV) (HV = 9, 15, and 21 mol%) are similar to the corresponding spectra of PHB. There are two bands at 97 and 82 cm(-1) which are assigned to a spring type vibrational mode of the helical structure and to the mode reflecting the intermolecular interaction (CH3 center dot center dot center dot O=C hydrogen bond), respectively. The low-frequency Raman and terahertz spectra of P(HB-co-HV) with HV content of 66 and 88.6 mol% show bands at 91 and 78 cm(-1). The 78 cm(-1) band is assigned to the intermolecular interaction (CH2 center dot center dot center dot O=C hydrogen bond). In contrast to the 97 cm(-1) band of P(HBco-HV) with the low HV content, the 91 cm(-1) band of P(HB-co-HV) with the high HV content show a temperature-dependent shift by 4 cm(-1). Thus, although it seems that both 97 and 91 cm(-1) bands are due to spring-like vibrational modes, the nature of two vibrational modes seems to be significantly different, reflecting the difference in the intermolecular interaction. (C) 2017 Elsevier Ltd. All rights reserved.