Organosilanes were grafted on thermoplastic polyurethanes (TPUs) using an internal mixer or reactive extrusion. The reaction described here proceeds in two stages, with the help of a diisocyanate as a coupling agent to create allophanate junctions between TPU chains and an aminosilane. Such materials could then be crosslinked by a hydrolytic condensation mechanism, resulting in increased thermomechanical properties. The starting TPU was at first carefully characterized. Using the plastograph, the different stages (reactions between TPU and diisocyanate, isocyanate-grafted TPU and aminosilane, but also between TPU and aminosilane alone) were then studied separately, together with the influence of several essential parameters (time, temperature, rotating speed, etc.). Optimal conditions for a well-controlled grafting process were defined, and the different ways of crosslinking the TPU chains were compared.