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Synopsis The catalytic formation of hydroxyl radicals in oxidative hair colourant systems in the presence of added copper ions was measured and quantified using a colorimetric probe N, N′-(5-nitro-1,3-phenylene)bisglutaramide. Also monitored in the same experiments was the decomposition of hydrogen peroxide. The first set of experiments was performed using aqueous model solutions containing the key oxidant actives in a hair colourant, ammonium hydroxide and hydrogen peroxide at p H 10, with added copper and calcium ions. The second set of experiments was performed in the presence of hair containing different levels of copper in conditions very close to those found during hair colouring. Both sets of experiments demonstrate the ability of copper ions to trigger the formation of hydroxyl radicals and catalyse the decomposition of hydrogen peroxide. The ability of chelants ethylenediamine tetraacetic acid (EDTA) and N, N′-ethylenediamine disuccinic acid ( EDDS) to moderate the flux of hydroxyl radicals formed in solution systems was demonstrated in the presence of copper ions alone. However, only EDDS was successful in the presence of both calcium and copper ions. This was confirmed in the hair experiments where again only EDDS was successful at preventing hydroxyl radical formation where hair is added as the source of copper and calcium ions. These results are explained using metal speciation modelling and demonstrate the importance of the chelant to be able to specifically bind and prevent the one-electron redox chemistry of copper in the presence of high levels of calcium ions as found in hair. The formation of hydroxyl radicals during the colouring process was shown to lead to hair structure damage as measured by protein loss. EDDS was demonstrated to significantly reduce cuticle damage by suppressing the formation of the hydroxyl radicals in systems with realistic concentrations of calcium and copper. [ABSTRACT FROM AUTHOR]