Abstract: A carbon monoxide-contaminated breathable gas (that is a gas or gas mixture containing sufficient oxygen to support human life) is purified by bringing it into contact with a bed of a catalyst, whereby to oxidize carbon monoxide to carbon dioxide and reduce the carbon monoxide contamination of gas, the catalyst being a promoted tin IV oxide-support precious metal catalyst in which the promoter is one or more metals of group 1b, 3b, 7bb and 8 of the Periodic Table (especially copper, nickel, manganese, silica or lanthanum) and the precious metal is platinum, palladium, rhodium, iridium or ruthenium.