The invention concerns a method for the cosmetic treatment of keratin fibres preferably hair comprising at least the steps consisting of: • a) applying to said keratin fibres a composition containing at least one substance chosen from fatty bodies non silicone polymers and surfactants • b) applying a mechanical tension to said keratin fibres and • c) exposing said keratin fibres 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 vapour form in contact with the keratin fibres and without the keratin fibres completely drying out during the entire exposure to microwaves • the solvent or solvents in vapour form being generated entirely by the evaporation of at least one compound present before the microwaves are emitted in contact with the keratin fibres • step a) taking place prior to step c).
