The greatest amounts of cyanide-containing wastes are produced by precious metals milling operations, the electroplating industry and coal processing or coking effluents processes. Because of high toxicity and to comply with federal and state regulations, the treatment of wastewater is required before safe discharge of cyanide wastes. In this regard, the gas-sparged hydrocyclone (GSH) has been tested as a reactor for the treatment of cyanide solutions for cyanide destruction by oxidation with the use of chlorine dioxide gas (ClO2). The results show oxidation efficiencies of free cyanide approached 99% at all pH values in 5 min. The use of NaCl was also considered for the generation of chlorine dioxide. Excellent performance appears to offer operational and cost advantages over conventional processes.