The process for increasing the adhesion of a reinforcing inorganic material in an organic matrix comprises: subjecting a dry load of particulate or fibrous reinforcing inorganic material presenting hydroxyls in its surface, to a first surface treatment with a coupling agent of the siloxane type containing amine groups, dissolved in an organic solvent free of water and with an acid pH and, after a curing step, to a new surface treatment with at least one coupling agent of the siloxane type dissolved in a solution containing an organic solvent and water, to be subjected to a new curing step. The reinforcement material thus treated may then be mixed to a load of organic material defined by a polymer or by a monomer, in order to obtain a composite mixture to form a composite polymeric material which may present the desired final form or a raw form, to be ground or pelletized into a particulate form for posterior processing.