A cosmetic composition for relaxing keratin fibres, comprising an alkaline agent, a fatty phase made up of fatty substance(s), the amount of fatty substance being greater than or equal to 52% by weight, relative to the total weight of said composition, and at least one surfactant in an amount of greater than or equal to 5% by weight, relative to the total weight of said composition. The application also relates to a method for relaxing keratin fibres using this composition, and also to the use of this composition for relaxing keratin fibres.
