The invention relates to a process for the preparation of a composition to be extruded comprising a polymer and a filler of solid particles comprising a stage of mixing the filler of solid particles with particles or granules of the said polymer, if appropriate in the form of an aqueous suspension. The invention is particularly simple, and the final material is in better mechanical properties than those obtained by direct incorporation of the solid filler in the polymer or copolymer. This improvement is very substantial, in particular with regard to the impact strength of the final material. The invention also relates to a polypropylene-based composition comprising a filler of solid particles, such as kaolin, exhibiting a high flexural modulus and a good impact strength.