The interface in polyhydroxybutyrate/PAN- and polyhydroxybutyrate/PET fibres model composites has been investigated by polarizing optical and scanning electron microscopy. Under certain melting/cooling conditions in PAN fibre system a transcrystalline interphase is clearly observed, whereas in PET/PHB model composites the transcrystallization phenomenon occurs only when fibres with significant surface roughness are used.