A method of decontaminating a metal surface, wherein the metal surface is located on a component within a nuclear plant, in particular within a coolant system of a nuclear power plant, and is covered with a metal oxide layer containing radioactive substances, and wherein the method comprises the following steps: a) an oxidation step in which the metal oxide layer is contacted with an aqueous oxidation solution comprising an oxidant; b) a decontamination step wherein the metal oxide layer treated in the oxidation step is contacted with an aqueous solution of an organic acid to dissolve the metal oxide layer, thereby forming a decontamination solution that contains the organic acid as well as metal ions and the radioactive substances, and wherein the decontamination solution is passed over an ion exchanger to immobilize the metal ions and the radioactive substances; wherein in step b) an oxidant selected from the group consisting of oxygen, air, hydrogen peroxide and ozone is dosed into the decontamination solution, wherein the decontamination solution contains both nickel ions and iron ions and wherein the nickel ions, in the decontamination step, are removed earlier from the decontamination solution than the iron ions, and wherein the decontamination solution is passed over a cation exchanger material, binding the nickel ions on the cation exchanger material, while the iron ions remain in the decontamination solution in the form of an anionic Fe complex. a) an oxidation step in which the metal oxide layer is contacted with an aqueous oxidation solution comprising an oxidant; b) a decontamination step wherein the metal oxide layer treated in the oxidation step is contacted with an aqueous solution of an organic acid to dissolve the metal oxide layer, thereby forming a decontamination solution that contains the organic acid as well as metal ions and the radioactive substances, and wherein the decontamination solution is passed over an ion exchanger to immobilize the metal ions and the radioactive substances; wherein in step b) an oxidant selected from the group consisting of oxygen, air, hydrogen peroxide and ozone is dosed into the decontamination solution, wherein the decontamination solution contains both nickel ions and iron ions and wherein the nickel ions, in the decontamination step, are removed earlier from the decontamination solution than the iron ions, and wherein the decontamination solution is passed over a cation exchanger material, binding the nickel ions on the cation exchanger material, while the iron ions remain in the decontamination solution in the form of an anionic Fe complex. wherein in step b) an oxidant selected from the group consisting of oxygen, air, hydrogen peroxide and ozone is dosed into the decontamination solution, wherein the decontamination solution contains both nickel ions and iron ions and wherein the nickel ions, in the decontamination step, are removed earlier from the decontamination solution than the iron ions, and wherein the decontamination solution is passed over a cation exchanger material, binding the nickel ions on the cation exchanger material, while the iron ions remain in the decontamination solution in the form of an anionic Fe complex.