Abstract: Carbon-containing compounds contained in a gas, such as polluted air or an industrial flue gas, or in a liquid, such as an aqueous solution or dispension, e.g. sewage liquid, contaminated drinking water or waste electroplating baths, are decomposed by being subjected to radiation of a wave length of about 20 to 600 nm in the presence of water and oxygen in excess of the stoichiometric amount required for complete oxidation. A catalytic quantity of hydrogen chloride may also be present. A suitable apparatus is also provided, comprising a chamber containing a source of radiation, a gas inlet and outlet, and means for spraying liquid into said chamber.