The change of molecular mobility during the phase transition of PHB/3-C1-PHB copolymer (5:1) from the orthorhombic modification II into an orthorhombic modification which is close to hexagonal chain packing was investigated by means of2H-NMR. It is shown that a flipping of the benzene rings takes place at temperatures far below the transition temperature. The phase transition starts after more than 60% of the benzene rings are flipping. Even in the newly formed phase close to the hexagonal chain packing, at temperatures up to 300°C, free rotation of the benzene rings can be excluded.