The effects of the rare earths in steels on the plasma nitriding, plasma sulphur-carbonitriding and gas carburising have been studied. An atomistic model based on electron affinities is proposed for the catalytic effect of the rare earths. It is pointed out that the rare earths accelerate the diffusion of carbon and nitrogen atoms, and the formation of nitride, carbide and carbonitride through direct and indirect actions. Therefore, the rare earths in steels have a significant catalytic effect during those treatments, and make the treated cases harder.