Blends of high density polyethylene (HDPE) and polypropylene (PP) with different biodegradable additives have been subjected to an outdoor soil burial test. The effect of the degradation process on the structural and morphological properties of the samples has been studied by thermogravimetry, differential scanning calorimetry, and dynamic-mechanical spectroscopy. The thermogravimetric results show that the additive is more affected by the degradation process than the polymeric matrix. Changes both in the crystalline morphology and the activation energies of the relaxation processes take place in different stages, and can be described using polynomial equations. These changes occur on different time scales depending on the additive used.