Process for dyeing keratin fibres, comprising steps i) and ii) below: i) applying to the fibres at least one cationic direct dye bearing a disulfide function, a thiol function or a protected thiol function, of formula: A- p -C sat -S-U salts thereof with an organic or mineral acid, optical or geometric isomers thereof, tautomers thereof, and solvates thereof, in which formula: • U represents a radical chosen from: a) - S-C' sat -(X') p' -A'; and b) -Y; • A and A', which may be identical or different, represent a radical containing at least one cationic chromophore; • Y represents i) a hydrogen atom; or ii) a thiol-function protecting group; • X and X', which may be identical or different, represent a linear or branched, saturated or unsaturated divalent C 1 -C 30 hydrocarbon-based chain, optionally interrupted and/or optionally terminated at one or both of its ends with one or more divalent groups or combinations thereof chosen from: -N -, -N + -, -O-, -S-, -CO-, -SO 2 - with R, which may be identical or different, chosen from a hydrogen and a C 1 -C 4 alkyl, hydroxyalkyl or aminoalkyl radical; an aromatic or non-aromatic, saturated or unsaturated, fused or non-fused (hetero)cyclic radical optionally comprising one or more identical or different, optionally substituted heteroatoms; • p and p', which may be identical or different, are equal to 0 or 1; • C sat and C' sat, which may be identical or different, represent an optionally cyclic, optionally substituted linear or branched C 1 -C 18 alkylene chain; and ii) applying water vapour having a temperature of greater than 80 °C to the fibres; steps i) and ii) being able to be performed together or separately; it being understood that the process does not use a reducing agent. i) applying to the fibres at least one cationic direct dye bearing a disulfide function, a thiol function or a protected thiol function, of formula: A- p -C sat -S-U salts thereof with an organic or mineral acid, optical or geometric isomers thereof, tautomers thereof, and solvates thereof, in which formula: • U represents a radical chosen from: a) - S-C' sat -(X') p' -A'; and b) -Y; • A and A', which may be identical or different, represent a radical containing at least one cationic chromophore; • Y represents i) a hydrogen atom; or ii) a thiol-function protecting group; • X and X', which may be identical or different, represent a linear or branched, saturated or unsaturated divalent C 1 -C 30 hydrocarbon-based chain, optionally interrupted and/or optionally terminated at one or both of its ends with one or more divalent groups or combinations thereof chosen from: -N -, -N + -, -O-, -S-, -CO-, -SO 2 - with R, which may be identical or different, chosen from a hydrogen and a C 1 -C 4 alkyl, hydroxyalkyl or aminoalkyl radical; an aromatic or non-aromatic, saturated or unsaturated, fused or non-fused (hetero)cyclic radical optionally comprising one or more identical or different, optionally substituted heteroatoms; • p and p', which may be identical or different, are equal to 0 or 1; • C sat and C' sat, which may be identical or different, represent an optionally cyclic, optionally substituted linear or branched C 1 -C 18 alkylene chain; and • U represents a radical chosen from: a) - S-C' sat -(X') p' -A'; and b) -Y; a) - S-C' sat -(X') p' -A'; and b) -Y; • A and A', which may be identical or different, represent a radical containing at least one cationic chromophore; • Y represents i) a hydrogen atom; or ii) a thiol-function protecting group; • X and X', which may be identical or different, represent a linear or branched, saturated or unsaturated divalent C 1 -C 30 hydrocarbon-based chain, optionally interrupted and/or optionally terminated at one or both of its ends with one or more divalent groups or combinations thereof chosen from: -N -, -N + -, -O-, -S-, -CO-, -SO 2 - with R, which may be identical or different, chosen from a hydrogen and a C 1 -C 4 alkyl, hydroxyalkyl or aminoalkyl radical; an aromatic or non-aromatic, saturated or unsaturated, fused or non-fused (hetero)cyclic radical optionally comprising one or more identical or different, optionally substituted heteroatoms; -N -, -N + -, -O-, -S-, -CO-, -SO 2 - with R, which may be identical or different, chosen from a hydrogen and a C 1 -C 4 alkyl, hydroxyalkyl or aminoalkyl radical; an aromatic or non-aromatic, saturated or unsaturated, fused or non-fused (hetero)cyclic radical optionally comprising one or more identical or different, optionally substituted heteroatoms; • p and p', which may be identical or different, are equal to 0 or 1; • C sat and C' sat, which may be identical or different, represent an optionally cyclic, optionally substituted linear or branched C 1 -C 18 alkylene chain; and ii) applying water vapour having a temperature of greater than 80 °C to the fibres; steps i) and ii) being able to be performed together or separately; it being understood that the process does not use a reducing agent.