The invention relates to a process for dyeing and/or lightening keratin fibres such as the hair, using i) at least one cationic direct dye bearing a disulfide, thiol or protected thiol function especially of formula (I) as defined in the description and ii) water vapour. The invention also relates to the use i) of at least one cationic direct dye bearing a disulfide, thiol or protected thiol function combined with ii) water vapour, for dyeing and lightening keratin fibres, and to the use of water vapour for grafting onto keratin fibres dyes bearing a disulfide, thiol or protected thiol function. The process used and the use of thiol, protected thiol or disulfide dyes combined with water vapour makes it possible especially to obtain a long-lasting coloration on keratin fibres without the use of a reducing agent and without odour.
