The development of a reactive pultrusion process is presented by considering the advantages of an in situ polymerisation of thermoplastic lauryllactam monomers. The specifics of the reaction injection unit, of the fibre-drying oven and of different pultrusion dies are given. Models were developed for the optimisation of the pultrusion process and for each processing step, namely: impregnation, polymerisation and cooling. Their individual influence on the overall pultrusion line speed was established. Heat transfer equations were solved in association with polymerisation kinetics and rheology results. Temperature profiles, conversion degrees, and viscosity changes were predicted versus composite part thickness and versus the position along the pultrusion line. Optimal mould lengths can thus be determined for optimum impregnation, polymerisation and cooling. A processing window has been defined in terms of pultrusion line speed and mould temperature. These results demonstrated with the production of profiles that reaction injection pultrusion of PA12 composites can be envisaged at larger scale.