A method is provided for removing carbonyl sulfide out of a H.sub.2 O-containing fluid stream, such as petrochemical vent gas, air, carbon dioxide, nitrogen and hydrocarbons, by determining how much carbon monoxide is present in the fluid stream; adding carbon monoxide to the fluid stream to obtain a carbon monoxide to carbonyl sulfide volume ratio of at least 1:1 and to also convert the carbonyl sulfide into hydrogen sulfide and carbon dioxide, and passing the hydrogen sulfide containing fluid through a hydrogen sulfide removal system, so as to obtain a fluid substantially free from carbonyl sulfide and hydrogen sulfide.