Abstract: A process for chemically modifying a polymeric part in order to provide it with flame retardancy properties or to enhance the properties, the process comprising a step of reacting at least one polymeric part comprising at least one polymer of which the reactive groups comprise amine groups and/or hydroxyl groups with a flame retardant compound comprising at least one group that reacts, by nucleophilic substitution or nucleophilic addition, with some or all of the amine groups and/or hydroxyl groups of the polymer or polymers, the reaction being carried out with the compound in gaseous form.