Nanocomposites are expected to exhibit new and improved properties when compared to their microcomposite counterparts. By lowering the particle size to nanodimensions (<100 nm), the special effects in polymer composites appear. In this study we compared the properties of composites filled with micro- and nano-sized calcium carbonate (CaCO3) filler particles in poly (vinyl acetate) (PVAc) matrix. The morphology of the composite was found to be responsible for the composite properties. The filler nanoparticles are dispersed in the matrix in the form of a 'net-like' structure, contrary to microparticles, which are dispersed as 'islands' in the matrix. The other systems investigated in this study were based on polyacrylate (PA) copolymer matrices filled with layered kaolin filler, which is well suited for creating nanocomposites. Mathematical models were used to quantify the interfacial interactions in the composites under investigation. Improved mechanical properties are obtained where there is a strong interfacial bond between the matrix and the filler. It seems that a key characteristic of the nanocomposites is the formation of a three-dimensional interphase with a significant amount of matrix with restricted chain mobility. The restricted molecular mobility in PVAc/CaCO3 nanocomposites resulted in changes of relaxation behavior, i.e. in the appearance of a second transition above the T. but only at large enough loading.