Polyurethane (PU) and poly(vinyl acetate) (PVAc) are partially miscible polymers. The aim of this study was to investigate the influence of untreated and surface pretreated calcium carbonate (CaCO3) on the interphase properties of PU/PVAc polymer blends with possible compatibilization effects when the conditions of effective adhesion are achieved. With the aim to modify interface between filler and matrices, CaCO3 was pretreated by grafting PVAc on the filler surface using Co-60 gamma-ray irradiation polymerization of vinyl acetate. A series of the PU/PVAc blends without and with the addition of CaCO3 were prepared using a Brabender plasticorder at 140 degrees C. Surface characterization of the PU and PVAc polymers as well as the untreated and surface pretreated CaCO3 fillers was carried out by determination of the surface free energy. The adhesion between CaCO3 and phases in the polymer blends was predicted on the basis of the calculated adhesion parameters (interfacial free energy, spreading coefficient and work of adhesion) obtained from the surface free energy of components. Thermal and mechanical properties were investigated by tensile strength measurements and differential scanning calorimetry (DSC). The adhesion parameters were correlated with the mechanical and thermal properties as well as phase morphology observations.