A method for styling, in particular for straightening, keratin fibers, in particular human hair, in which (i) a styling agent comprising carbocysteine and/or its salts is applied to the keratin fibers and left thereon, (ii) the fibers, after a contact time, - are not rinsed, - are optionally dried, (iii) the fibers are mechanically deformed under the influence of heat, the fibers being subjected to heat treatment at a temperature of from 80°C to 280°C, characterized in that, in step (i), a styling agent is applied which contains, based on its weight, - from 0.1 to 7.5 wt.% carbocysteine, - from 40 to 92.5 wt.% water, - from 0.15 to 5 wt.% quaternary ammonium compound(s) selected from the group of i) tetraalkylammonium halides and/or ii) esterquats and/or iii) quaternary imidazolines of formula (Tkat2) in which the functional groups R, independently of one another, each represent a saturated or unsaturated, linear or branched hydrocarbon functional group having a chain length of 8 to 30 carbon atoms and A represents a physiologically acceptable anion, and/or iv) poly(methacryloyloxyethyltrimethylammonium) compounds and/or v) quaternized cellulose derivatives, in particular Polyquaternium-10, Polyquaternium-24, Polyquaternium-27, Polyquaternium-67, Polyquaternium-72 and/or vi) cationic alkyl polyglycosides and/or vii) cationized honey and/or viii) cationic guar derivatives and/or ix) polymeric dimethyldiallylammonium salts and copolymers thereof with esters and amides of acrylic acid and methacrylic acid, in particular Polyquaternium-7 and/or x) copolymers of vinylpyrrolidone with quaternized derivatives of dialkylaminoalkyl acrylate and methacrylate, in particular Polyquaternium-11 and/or xi) vinylpyrrolidone-vinylimidazolium methochloride copolymers, in particular Polyquaternium-16 and/or xii) quaternized polyvinyl alcohol and/or xiii) Polyquaternium-74, xiv) cationic alkyl oligoglucosides and/or xv) Polyquaternium-71 and mixtures thereof. (i) a styling agent comprising carbocysteine and/or its salts is applied to the keratin fibers and left thereon, (ii) the fibers, after a contact time, - are not rinsed, - are optionally dried, - are not rinsed, - are optionally dried, (iii) the fibers are mechanically deformed under the influence of heat, the fibers being subjected to heat treatment at a temperature of from 80°C to 280°C, - from 0.1 to 7.5 wt.% carbocysteine, - from 40 to 92.5 wt.% water, - from 0.15 to 5 wt.% quaternary ammonium compound(s) selected from the group of i) tetraalkylammonium halides and/or ii) esterquats and/or iii) quaternary imidazolines of formula (Tkat2) in which the functional groups R, independently of one another, each represent a saturated or unsaturated, linear or branched hydrocarbon functional group having a chain length of 8 to 30 carbon atoms and A represents a physiologically acceptable anion, and/or iv) poly(methacryloyloxyethyltrimethylammonium) compounds and/or v) quaternized cellulose derivatives, in particular Polyquaternium-10, Polyquaternium-24, Polyquaternium-27, Polyquaternium-67, Polyquaternium-72 and/or vi) cationic alkyl polyglycosides and/or vii) cationized honey and/or viii) cationic guar derivatives and/or ix) polymeric dimethyldiallylammonium salts and copolymers thereof with esters and amides of acrylic acid and methacrylic acid, in particular Polyquaternium-7 and/or x) copolymers of vinylpyrrolidone with quaternized derivatives of dialkylaminoalkyl acrylate and methacrylate, in particular Polyquaternium-11 and/or xi) vinylpyrrolidone-vinylimidazolium methochloride copolymers, in particular Polyquaternium-16 and/or xii) quaternized polyvinyl alcohol and/or xiii) Polyquaternium-74, xiv) cationic alkyl oligoglucosides and/or xv) Polyquaternium-71 i) tetraalkylammonium halides and/or ii) esterquats and/or iii) quaternary imidazolines of formula (Tkat2) in which the functional groups R, independently of one another, each represent a saturated or unsaturated, linear or branched hydrocarbon functional group having a chain length of 8 to 30 carbon atoms and A represents a physiologically acceptable anion, and/or iv) poly(methacryloyloxyethyltrimethylammonium) compounds and/or v) quaternized cellulose derivatives, in particular Polyquaternium-10, Polyquaternium-24, Polyquaternium-27, Polyquaternium-67, Polyquaternium-72 and/or vi) cationic alkyl polyglycosides and/or vii) cationized honey and/or viii) cationic guar derivatives and/or ix) polymeric dimethyldiallylammonium salts and copolymers thereof with esters and amides of acrylic acid and methacrylic acid, in particular Polyquaternium-7 and/or x) copolymers of vinylpyrrolidone with quaternized derivatives of dialkylaminoalkyl acrylate and methacrylate, in particular Polyquaternium-11 and/or xi) vinylpyrrolidone-vinylimidazolium methochloride copolymers, in particular Polyquaternium-16 and/or xii) quaternized polyvinyl alcohol and/or xiii) Polyquaternium-74, xiv) cationic alkyl oligoglucosides and/or xv) Polyquaternium-71