The use of rigid filters to toughen polymers has received considerable attention in recent years. The role of the rigid particle here is that of debonding, at some stage, from the matrix, thus triggering dilatational processes similar to those observed in rubber-toughened polymers. The role of particle size in these rigid filled composites has not been studied in great detail. In this work, volume strain measurements were carried out on a series of particulate composites based on polypropylene filled with calcium carbonate (CC) particles with average diameters of 0.07, 0.7, and 3.5 mum and filler volume fractions ranging from 0.05 to 0.30. The experimental results have shown a strong particle size effect. A model is proposed to take this effect into account, based upon the formation of an immobilized layer of polymer on the surface of the filler particles. The experimental results are consistent with a surface layer of 15-25 nm. The results are discussed in relation to the fracture behavior of these composites reported earlier. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 925-935, 2004.