Composites of isotactic polypropylene with various contents of white clay or titanium dioxide TiO2 were prepared by extrusion molding. The extruded composites were melt-pressed at two different temperatures, and, thereafter, either slowly cooled, or quenched to room temperatures. It is shown that the structure of all the samples, as revealed by wide-angle X-ray scattering and small-angle X-ray scattering (SAXS), depends on the processing conditions. The lack of SAXS maxima in the composites suggests that the presence of the microadditives hinders the stacking of iPP lamellae. Furthermore, the microindentation hardness H in the slowly cooled composites is influenced by the type and amount of the filler used. However, in the quenched samples H depends only on the amount of the filler used, and not on its type. In case of the quenched iPP/clay composites, the relationship between H and the Young's modulus E is found to be H/E approximate to 0.12, in good agreement with Struik's theoretical predictions of se approximate to E/30, in consonance with results previously obtained for a series of polyethylene samples with different morphology. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012