A process is disclosed for preparing chlorodifluoracetyl chloride from 1,1,-difluor-2,2-dichlorethylene, dichloracetyl chloride from trichlorethylene or 1,1,2,2-tetrachlorethane and trichloracetyl chloride from tetrachlorethylene. Said starting compounds are continuously reacted in the gaseous phase with oxygen to obtain a photochemical oxidation reaction, chlorine being optionally added as a sensitiser. When chlorine is added, the reaction mixture is irradiated with light with a wavelength lambda >/= 280 nm. The reaction is preferably carried out without pressure. Particularly high yields with high selectivity are reached by using doped high-pressure mercury-vapour lamps.