Changes of the unit cell parameters in the crystalline phase of the poly(beta-hydroxy-butyrate-co-valerate) (PHB/HV) containing 0, 6, 15 and 22 mol% HV, during heating from room temperature to the melting point, were studied. It was found that unit cell expansion occurred with increasing HV monomer concentration from 0 mol% to 22 mol%. The increase in lattice parameters a and b are almost linear with increasing concentration and the concentrational expansion coefficients in the direction a and b are about 6 x 10(-4) (mol%HV)(-1) and 3 x 10(-4) (mol%HV)(-1), respectively. The parameter c (fibre axis) remains unchanged. Temperature change causes unit cell expansion characterized by significant change of the parameter a only, with the linear coefficient of thermal expansion of about 1.3 x 10(-4) degrees C-1.