Semi-crystalline polyhydroxybutyrate-co-hydroxyvalerate samples were stored at a range of temperatures between 22 and 150 °C, for up to 672 hours. The secondary crystallisation process was monitored using differential scanning calorimetry (DSC) and infrared spectroscopy, and crystallinity was observed to increase over time at storage temperatures of 20 °C and upwards. Higher storage temperatures produced a greater extent of change within the timescales of this work, indicating a rate effect of temperature on secondary crystallisation. DSC studies also revealed an increase in the melting temperature (Tm), and dynamic mechanical thermal analysis revealed an increase in the glass transition temperature (Tg), in accordance with increasing crystallinity. The corresponding effects on the mechanical properties were analysed using tensile testing. Generally, it was found that with increases in crystallinity there was an increase in the ultimate tensile strength, a decrease in the elongation to break, and therefore an increase in the Young’s Modulus. The increasing embrittlement of samples was most evident during storage at 150 °C, where subsequent decreases in crystallinity, Tm, Tg, mass, molecular weight (Mw), and sample discolouration over time were evidence of an interplay between the secondary crystallisation process and the thermal degradation process, active below the Tm.