A process for treating effluent wherein an oxidizing agent in the effluent is decomposed on contact with a catalyst bed. The oxidizing agent can be a pollutant present in the effluent, for example hypochlorite in the effluent from chlorine-producing brine electrolysis cells or from the scrubbing of chlorine gas. In an alternative the effluent may contain oxidizable pollutants and the oxidizing agent is purposefully added thereto. Passage over the catalyst bed catalyzes the decomposition of the oxidizing agent and oxidizable pollutants.The catalyst bed comprises an intimate mixture of an oxide of Group III, nickel oxide and, optionally, cobalt oxide, supported on pieces of a non-porous substantially inert material.