The invention relates to a cosmetic process for treating keratin fibers, preferably the hair, comprising at least the steps consisting in: a) applying to said keratin fibers a composition containing at least one substance chosen from fatty substances, non-silicone polymers and surfactants, b) applying a mechanical tension to said keratin fibers, and c) exposing said keratin fibers under mechanical tension to microwaves, at a pressure ranging from 50,000 to 250,000 Pa, in the presence of at least one solvent in vapor form on contact with the keratin fibers and without there being complete drying of the keratin fibers throughout the entire exposure to the microwaves, the solvent(s) in vapor form being entirely generated by evaporating at least one compound present, before emission of the microwaves, on contact with the keratin fibers, step a) taking place prior to step c).