Abstract: A catalyst for decomposing an oxidising agent, such as hypochlorite ions, in an effluent comprises an intimate mixture of at least one finely divided Group IIA oxide and finely divided nickel oxide, and/or hydrates thereof, supported on pieces of a non-porous substantially inert material. Some of the nickel may be replaced by cobalt. The Group IIA oxide forms 0.5 to 50% by weight of said intimate mixture. The nickel and Group IIA oxides may be applied by contacting the support with a solution of a suitable salt, eg nitrates. Where the support is a Group IIA compound, eg magnesia, the Group IIA oxide may be incorporated into the intimate mixture by contacting the support with an acid solution of the nickel salt. In use the nickel oxide as oxidised to a higher oxidation state than NiO.