Process for the preparation of polyalkenyl acylating agents which comprises: a. reacting at a temperature higher than 180° C. a reactive polyalkene, having a number average molecular weight Mn ranging from 500 to 5000 and having a content of terminal vinylidene groups greater than or equal to 30%, with an enophile; b. thermally carrying out the reaction for a time sufficient for having a conversion of the terminal vinylidene groups higher than 15%; c. completing the reaction always under heat in the presence of a reaction accelerator consisting of a Lewis acid selected from a tin, zinc, aluminum or titanium halide.