This paper investigates the effect of the interphase properties and the interfacial interactions between matrix and filler on mechanical properties of precipitated calcium carbonate (PCC)-polypropylene nanocomposites. PCC particles were coated with stearic acid (SA). The weight ratio of SA on the particles (wSA) ranged from 0 to 0.135 g SA/g PCC. The introduction of PCC particles resulted in an increase in stiffness and yield stress compared with the pristine polymeric matrix and, at the same time, it increased the impact resistance. The maximum improvement in the impact behaviour was achieved for the composites with wSA = 0.045 corresponding to the theoretical monolayer ratio. A decrease in interfacial interactions between monolayer coated PCCs and the matrix with respect to the uncoated particles was observed by using a semi-empirical equation developed by Pukanzky. The low degree of interfacial interactions between particulate filler and matrix allows a matrix-particle debonding phenomenon, as shown by scanning electron microscopy analysis. Extensive plastic deformations were evident as well, promoting an improvement in toughness. The thickness of the interphase between particles and matrix was evaluated by using the Shen-Li model which is based on the hypothesis of a non-homogeneous interphase. It results that the thickness increased in the order uncoated < monolayer coated < 3% SA coated congruent to 13.5% SA coated particles. The thinner and stronger interphase found for the composite with uncoated particles can be explained with the high interaction between matrix and filler and the consequent low mobility of the polymeric chains. (C) Koninklijke Brill NV, Leiden, 2010