The interface in particulate filled polymer systems determine to a large extent, the mechanical and morphological properties of composites and polymer blends. The present contribution describes surface free energy measurements and calculations of adhesion parameters at the interface in poly(vinyl acetate) (PVAc) and polyurethane (PILI) composites as well as in styrene acrylonitrile/ethylene propylene diene monomer (SAN/EPDM) blends filled with calcium carbonate (CaCO3) nanofillers. The effects that a surface pretreatment of the filler with either silane or stearic acid had on the interface been investigated and related to the resulting properties of the composites and filled polymer blend. The optimal adhesion in the particulate composites and blends was determined by the minimal interfacial energy and optimal coefficient of wetting. The conditions of the optimal adhesion with filler location at the interface, predicting by the thermodynamical models, correlated well with the improvement of the morphological and mechanical properties.