The present invention relates to a composition for dyeing keratin fibres, comprising: • a) one or more liquid fatty substances • b) one or more amphoteric surfactants of formula (I) below: • R a-C(0)-NH-CH 2-(CH 2) n-Ν(Β)(Β) (I) in which : •. B represents the group -CH 2-CH 2-O-X •. B represents the group -(CH 2) ZY< with z = 1 or 2 • ■ X represents the group -CH 2-C(O)OH, -CH 2-C(O)OZ, -CH 2- CH 2-C(O)OH or -CH 2-CH 2-C(O)OZ, or a hydrogen atom • ■ Y represents the group -C(O)OH, -C(O)OZ", -CH 2-CH(OH)-S0 3H or the group - CH2-CH(OH)-S0 3-Z" •. Z and Z" represent, independently of each other, a cationic counterion derived from an alkali metal or alkaline-earth metal, such as sodium, an ammonium ion or an ion derived from an organic amine •. R arepresents a C 10-C 30alkyl or alkenyl group derived from an acid R a-C(O)OH, which is preferably present in copra oil or in hydrolysed linseed oil, an alkyl group, especially a C 17group and its iso form, or an unsaturated C 17group and • ■ n represents an integer ranging from 1 to 10 and preferably from 1 to 5, • ■ or quaternized forms thereof • c) one or more thickening polymers chosen from polymers bearing sulfonic unit(s), polymers bearing sugar unit(s) and mixture thereof • d) one or more oxidation dye precursors and • e) one or more chemical oxidizing agents, The present invention also relates to a process using this composition. The present invention relates to a composition for dyeing keratin fibres, comprising: a) one or more liquid fatty substances b) one or more amphoteric surfactants of formula (I) below: Ra-C(O)-NH-CH2-(CH2)n-N(B)(B) (I) in which: B represents the group -CH2-CH2-O-X B represents the group -(CH2)ZY, with z = 1 or 2 X represents the group -CH2-C(O)OH, -CH2-C(O)OZ, -CH2-CH2-C(O)OH or -CH2-CH2-C(O)OZ, or a hydrogen atom Y represents the group -C(O)OH, -C(O)OZ", -CH2-CH(OH)-SO3H or the group -CH2-CH(OH)-SO3-Z" Z and Z" represent, independently of each other, a cationic counterion derived from an alka
