Apparent reactive coupling of CaCO3 to polypropylene (PP) was achieved with the application of aminofunctional silane coupling agents. The strong coupling effect could be attributed to increased CaCO3/silane and PP/silane interaction. A detailed study has shown that aminofunctional silanes bond strongly to the surface of the filler, but in the case of multilayer coverage also to the previous layers. Experiments were carried out to determine the mechanism of interaction between the silane coupling agents and the apolar polypropylene, which does not contain reactive groups. Model reactions followed by (Fourier Transform Infrared Spectroscopy) FTIR analysis demonstrated that during processing at elevated temperatures, oxidation of the polymer takes place in spite of the presence of stabilizers. Reactive carbonyl groups form as a result, which enter into chemical reactions with the amino functionality of the silane, resulting in strong amide bonds.