Abstract: A method for treating a cyanide-containing waste water by mixing the waste water with a treating agent containing a carbonyl compound such as formaldehyde and subjecting said mixture to an elevated temperature in an atmosphere containing oxygen to evaporate water and volatile materials therefrom, decompose the cyanide-containing component thereof and burn the decomposed cyanide component and other combustible materials to provide exhaust gases of substantially reduced cyano component. Metals contained in the waste water in the form of noxious compounds are also converted into harmless forms and recovered and no toxic components are released into the atmosphere or discharged as waste. An apparatus useful in carrying out the method is also disclosed.