A study is reported of the oxidation of several formazans by copper(II) ammine complexes in 45.5 wt% ethanol-water mixtures at constant temperature (25±0.1°C) and at constant ionic strength (0.09 M ammonium nitrate). The rate law found indicates that the mechanism for the initial stage of the oxidation is anlogous to the complex formation reaction of copper(II) with 1,3-diphenyl-5-o-carboxyphenylformazan. The change in rate constant for the oxidation of 1,5-diphenyl-3-carboxyformazan on variation of the weight percentage of ethanol is consistent with this mechanism. A comparison between the oxidation of the formazans by hexacynoferrate(III) and copper(II) revealed that in 45.5 wt% ethanol-water the ratio k(Cu2+/k(Fe(CN)63−) was nearly equal for the « red » formazans TPF and DPCpPf. The « yellow » formazans DPF and DPCF have different values for this ratio.