Samples of polypropylene and a high density polyethylene/polypropylene blend filled with different biodegradable additives (rice starch/iron oxide mixture, Bioefect 72000 and Mater-Bi AF05H) have been subjected to an outdoor soil burial test for 21 months. Changes on the morphology of the samples have been studied by differential scanning calorimetry. The degradation process also has been analyzed in terms of the mechanical behavior of the polymers. The three characteristic relaxation zones alpha, beta, and gamma (in order of decreasing temperature) of the dynamic-mechanical relaxation spectra of the samples have been characterized according to the Fuoss-Kirkwood equation. When two relaxations were overlapped, a deconvolution method was applied. It has been observed that the degradation process affects just to a small extent the amorphous phase of the polymers. That the beta relaxation is the most sensitive to the exposure time suggests that degradation starts in the crystalline-amorphous interface. However, the crystalline phase is also affected significantly by the degradation process. The mechanical results are in good agreement with the calorimetric measurements, proving that degradation takes place in two stages with different time scales depending on the additive used.