Blends of polypropylene (PP) and starch-based biodegradable materials were tested for biodegradability under soil burial test conditions. By means of thermogravimetric analysis, we evaluated the thermal stability of the PP matrix, of the additive and of the blends before and after the soil burial test. The kinetic parameters associated with thermal degradation were determined using integral isoconversional methods. We present a methodology that allows the kinetic triplet to be determined on the basis of non-isothermal experiments, and which gives results similar to those obtained in isothermal experiments. The kinetic parameters show the effect of degradation in soil on the blends studied. All the materials studied decompose by a type Rn mechanism in a nitrogen atmosphere and a type An mechanism in an oxygen atmosphere. The biodegradation was seen to affect the starch of the additive and not the PP matrix.