Injected polypropylene/talc composites were studied to evaluate the conditions leading to the formation of a mixed talc/polymer crystalline lamella percolating network and the influence of such a network on the nanocomposite mechanical properties. The talc was either conventional micrometer-sized (conventional talc) or submicrometer-sized particles (mu-talc). In the case of mu-talc, several talc fractions were studied, ranging from 3 to 30 wt %. The nanocomposite crystallinity was characterized with differential scanning calorimetry and wide-angle X-ray scattering. Talc was found to act as a nucleating agent, and only the alpha phase was detected. Through quantification on a Wilchinsky diagram, the talc particles were found to tie in the sample plane, the polypropylene crystalline lamellae being orthotropically distributed perpendicularly to the talc particles. The mechanical properties of the composites were tested in different directions by tensile and compression tests. The mechanical behavior of the composites confirmed the microstructural model. For low talc loadings, the composite moduli could not be well fitted by a law of mixtures. The large difference between the observed and predicted moduli was attributed to the formation of a mixed percolating network, including talc particles and polypropylene crystalline lamellae. At high talc loadings, when the mixed percolating network was completely formed, the reinforcement could well be described by parallel coupling, which indicated a classical reinforcement mechanism. Finally, the value of the critical talc fraction, at which the mixed percolating network was formed, was examined as a function of talc. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 3097-3105, 2009