Samples of polypropylene (PP) filled with a biodegradable additive marketed under the Bioefect trademark, were subjected to an outdoor soil burial test for 21 months. Samples were initially characterized by thermogravimetry. The kynetics of the thermal degradation of both the carbonated chains of PP and the additive have been studied by means of the Hirata differential method and the Broido integral method. Such analysis reveals that the additive is more affected by the degradation process than the PP matrix. Changes in the morphology of the samples with the exposure time have been analyzed by Differential Scanning Calorimetry, in terms of the crystalline content of PP and its lamellar thickness distribution. The beta- and alpha -relaxation zones of the dynamic mechanical relaxation spectra of both PP and pure Bioefect have been characterized using the Fuoss-Kirkwood equation and a deconvolution method. The analysis of the relaxation spectra shows that the interfacial and crystalline regions of the PP matrix are quite affected by the degradation process. On the other hand, it has also been found that changes in the crystallinity and the mechanical behavior of the samples take place in different stages. Such an evolution can be adequately represented by polynomial equations.