Polyhydroxyalkanoates (PHAs) are a class of biopolymers that are currently the subject of intensive research for various applications (packaging, consumer products, medical applications, etc.). It is known from synthetic polymers that all plastic materials show more or less pronounced autoxidation (aging induced by UV radiation, temperature, heavy metal ions, etc.). There is less knowledge as yet regarding the autoxidation behavior of biopolymers. The autoxidative behavior of medium chain length poly[(R)-3-hydroxyalkanoate] (mcl-PHA) was therefore investigated. mcl-PHA (co)polymers with amounts of 0, 10, 50, and 75 mol % of olefinic side chains with terminal double bonds were tempered at 60 degrees C in air for 3 months. After 1, 2, 4, 8, and 12 weeks, samples were removed and analyzed for changes in chemical and physical properties by sol-gel analysis (Soxhlet extraction), size exclusion chromatography (SEC), infrared analysis (IR), and gas chromatography/flame ionization detection (GC/FID). It became apparent that the content of double bonds greatly influences the autoxidation of mcl-PHA. A low amount of unsaturated moiety (0 and 10 mol %) resulted in chain scission, whereas samples with 50 and 75 mol % olefinic side chains showed cross-linking and became insoluble after a few weeks. Kinetic data of oxidation behavior were investigated by performing isothermal DSC experiments at elevated temperatures. The kinetic data combined with the experiment enabled the gelation time to be predicted and the shelf-life of mcl-PHA to be estimated. Because of the detected sensitivity of mcl-PHA regarding autoxidation, it is recommended that these biopolymers should be stored cold (at least -5 degrees C) and in an inert gas atmosphere or stabilized by suitable additives (antioxidants).