Preparing a reactive thermoplastic composition comprising a continuous phase which is made of at least one thermoplastic polymer and in which a discontinuous phase is dispersed which is made of at least one reactive chemical reagent of reinforcement can be immiscible with the thermoplastic polymer comprising group constituted by epoxy resins, polyorganosiloxanes having functional groups of silicon-hydride (SiH), and/or di- or polyisocyanates, comprises in situ grafting, branching and/or crosslinking, by mixing the phases with a shear rate greater than 10 2>s -> 1>, preferably >= 10 3>s -> 1>. Preparing a reactive thermoplastic composition comprising a continuous phase which is made of at least one thermoplastic polymer and in which a discontinuous phase is dispersed which is made of at least one reactive chemical reagent of reinforcement can be immiscible with the thermoplastic polymer comprising group constituted by epoxy resins, polyorganosiloxanes having functional groups of SiH, and/or di- or polyisocyanates, comprises in situ grafting, branching and/or crosslinking, by mixing the phases with a shear rate greater than 10 2>s -> 1>, preferably >= 10 3>s -> 1>, where at least one reinforcing agent is present on the chain of the thermoplastic polymer, so that the discontinuous phase is dispersed in the continuous phase in the form of nodules of average size in number less than 5 mu m with a maximum standard deviation of 0.5 mu m. An independent claim is included for the reactive thermoplastic composition, used to form a reactive system comprising group constituted by systems compatibilizing between phases, systems of sizing fibers in composites and systems for dispersing fillers in thermoplastic phases, where the composition is devoid of any compatibilizing agent between the phases.