In a process for chlorinating ketones which, apart from the carbonyl group, are inert in respect of triarylphosphine dichlorides, except for cyclopropyl methyl ketone, in which the ketones are reacted with a chlorinating agent in the presence of triarylphosphine oxides, the amount of triarylphosphine oxide is from 0.1 to 10 mol %, based on the amount of ketone. The ketones preferably have a least one CH-acid proton in the &agr; position to the carbonyl group.