The rheological data obtained for a poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV copolymer shows that the reduction in viscosity associated with degradation is very fast and is detectable even at temperatures close to the melting point. Size Exclusion Chromatography analysis of selected samples shows that, except for residence times below 15s, molecular mass distribution is substantially affected, shifting toward lower masses, and with an increase in oligomer compound content. These oligomers may act as plasticizers of the bioplastic.