The invention relates to a process for dyeing and/or lightening keratin fibers 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 vapor. 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 vapor, for dyeing and lightening keratin fibers, and to the use of water vapor for grafting onto keratin fibers 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 vapor makes it possible especially to obtain a long-lasting coloration on keratin fibers without the use of a reducing agent and without odor.