Abstract: Described herein are polyolefins functionalized with maleic anhydrid and at least one high-boiling ester of the latter, and a process for their preparation in high yields, with high degrees of functionalization, without the production of by-products of cross-linking or degradation. The process, which may be carried out either continuously or discontinuously, is characterized by the use of inorganic substances having a specific porosity, grain size, density and humidity, as vehicles of the functionalizing monomers; the presence of maleic ester contributes to enhancing the reaction efficiency. Likewise described herein is a double-screw extruder devised for carrying out the said process, and the use of products obtained as compatibilizing additives in the field of adhesive and reinforcing polymers.