A subject of the invention is a process for dyeing keratin fibres such as the hair, using at least one cationic direct dye bearing a disulfide, thiol or protected-thiol function and comprising at least one ammonium group bearing an aliphatic chain (I). Another subject of the invention is the use i) of at least one cationic direct dye bearing a disulfide, thiol or protected-thiol function (I) combined with ii) steam and/or a heating iron for straightening keratin fibres, for dyeing keratin fibres, and novel cationic direct dyes bearing a disulfide, thiol or protected-thiol function (I), and also a cosmetic composition comprising same. The process used and the use of the thiol, protected-thiol or disulfide dyes combined with steam makes it possible in particular to obtain a long-lasting colouration with a care valency on the keratin fibres.
