Process for dyeing keratin fibres, consisting in applying to said fibres one or more cationic direct dye(s) bearing a disulfide, thiol or 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 cationic or non-cationic chromophore, preferably a cationic chromophore, it being understood that at least one of the two chromophores bears at least one group bearing a linear or branched, saturated or unsaturated, C 10 -C 30 aliphatic chain; • 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 heteroatoms, identical or different, optionally substituted; • p and p', which may be identical or different, are equal to 0 or 1; • C sat and C' sat, identical or different, represent an optionally cyclic, optionally substituted, linear or branched, C 1 -C 18 alkylene chain. • 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 cationic or non-cationic chromophore, preferably a cationic chromophore, it being understood that at least one of the two chromophores bears at least one group bearing a linear or branched, saturated or unsaturated, C 10 -C 30 aliphatic chain; • 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 heteroatoms, identical or different, optionally substituted; -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 heteroatoms, identical or different, optionally substituted; • p and p', which may be identical or different, are equal to 0 or 1; • C sat and C' sat, identical or different, represent an optionally cyclic, optionally substituted, linear or branched, C 1 -C 18 alkylene chain.