A process for treating hair, especially human hair, comprising the steps of - application onto hair of a ready-to-use composition obtained by mixing the compositions A, B and C which are being kept separately until immediately before application onto hair at a weight ratio of A:B:C in the range of 1:2:0.2 to 1:1:1, - leaving it on the hair for a period of 1 min to 45 min, - rinsing it off from the hair, and - optionally washing the hair with a cleansing composition and drying, wherein the composition A is a composition comprising one or more alkalizing agents and having a pH ranging from 7.5 to 12, wherein the composition B is an aqueous composition comprising one or more oxidizing agents and has a pH in the range of 1.5 to 5, wherein the composition C comprises i. one or more carboxylic acids having three or more carboxyl groups and/or their salts selected from citric acid, ethylenediamine tetraacetic acid (EDTA), pyromellitic acid and glutamate diacetate, and ii. one or more additional organic acid and/or their salts having one or two carboxyl groups selected from the group of acetic acid, malic acid, lactic acid, glycolic acid, tartaric acid, formic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, and fumaric acid, wherein the composition C comprises the acids of i) and ii) and/or their salts at a total concentration of 10% to 100% by weight calculated to the total of composition C, wherein the ready-to-use composition has an alkaline pH in the range from 8 to 11 and comprises the acids and/or their salts at a total concentration in the range from 1% to 10% by weight calculated to the total of the ready-to-use composition. - application onto hair of a ready-to-use composition obtained by mixing the compositions A, B and C which are being kept separately until immediately before application onto hair at a weight ratio of A:B:C in the range of 1:2:0.2 to 1:1:1, - leaving it on the hair for a period of 1 min to 45 min, - rinsing it off from the hair, and - optionally washing the hair with a cleansing composition and drying, wherein the composition A is a composition comprising one or more alkalizing agents and having a pH ranging from 7.5 to 12, wherein the composition B is an aqueous composition comprising one or more oxidizing agents and has a pH in the range of 1.5 to 5, wherein the composition C comprises i. one or more carboxylic acids having three or more carboxyl groups and/or their salts selected from citric acid, ethylenediamine tetraacetic acid (EDTA), pyromellitic acid and glutamate diacetate, and ii. one or more additional organic acid and/or their salts having one or two carboxyl groups selected from the group of acetic acid, malic acid, lactic acid, glycolic acid, tartaric acid, formic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, and fumaric acid, wherein the composition C comprises the acids of i) and ii) and/or their salts at a total concentration of 10% to 100% by weight calculated to the total of composition C, wherein the ready-to-use composition has an alkaline pH in the range from 8 to 11 and comprises the acids and/or their salts at a total concentration in the range from 1% to 10% by weight calculated to the total of the ready-to-use composition. i. one or more carboxylic acids having three or more carboxyl groups and/or their salts selected from citric acid, ethylenediamine tetraacetic acid (EDTA), pyromellitic acid and glutamate diacetate, and ii. one or more additional organic acid and/or their salts having one or two carboxyl groups selected from the group of acetic acid, malic acid, lactic acid, glycolic acid, tartaric acid, formic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, and fumaric acid,