Abstract: When aqueous solutions of a metal cyanide complex are detoxified with hydrogen peroxide, a residue of cyanide remains, which in the case of nickel for example may be present as Ni(CN).sub.2. In a process according to the present invention the detoxification of aqueous alkaline solutions of a metal cyanide complex is carried out using a restricted amount of certain complexing agents together with the hydrogen peroxide. A particularly suitable example of such complexing agents is ethylenediaminotetraacetic acid. It is especially preferred to use the complexing agent in a mole ratio to metal of from 0.8:1 to 1.5:1, together with hydrogen peroxide in a mole ratio to cyanide of from 2:1 to 3:1. The process is well suited to the treatment of concentrated solutions of cyanide, in many cases at least 4000 ppm, for example those resulting from the stripping of nickel plating.