Disclosed is a method for the oxidative lightening and/or dyeing of keratinous fibers, more particularly of hair, wherein (a) a dye and/or lightening agent is applied to the keratinous fiber, said dye or lightening agent being obtained by mixing a composition (A), which contains at least one alkalizing agent, with a composition (B), which contains, in a cosmetic carrier, at least one oxidant, (b) within a period of one second to 24 hours after step (a), a post-treatment agent containing at least one salt of an isoalkylphosphoric acid ester, is applied to the keratinous fibers. Most preferably the isoalkylphosphoric acid ester salt is formed from the partial esterification of phosphoric acid with 2-ethylhexylalcohol and isotrideceth-8； the cation of the salt is, preferably, potassium or sodium. Most preferably, the post-treatment agent comprises； the isoalkylphosphoric acid ester, water, and a further hair-conditioning active (e.g. cetearyl alcohol)
