The thermal stability of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(HB-co-HV)] and its blends with poly(propylene glycol)s (PPGs) and castor oil (CO) is reported. The study includes the determination of the degradation kinetics of these materials and the analysis of the effects of the degradation on the mechanical properties and crystallization behavior. Spectroscopy (H-1-NMR, FTIR), differential scanning calorimetry (DSC), thermogravimetry, and tensile testing techniques are used for the experimental analysis. A chain-scission degradation mechanism is confirmed by the formation of vinyl groups. Two temperature ranges are investigated. In the range closest to the melting point, 100-200 degrees C, where the blend does not exhibit weight reduction, a fast and sensible loss of molecular weight and tensile strength was detected. The second temperature range, 200-400 degrees C, is characterized by mass loss by pyrolysis. In this range, different kinetic models of the degradation process are proposed. Polyalcohol addition produces opposite effects, while the addition of PPG enhances the degradation of P(HB-co-HV). When CO is added, the thermal stability of the blend increases. Mechanical properties of the blends before and after degradation were determined. The tensile modulus increases at the first step of degradation and decreases with the degradation time.