Generally, very little attention is given to the metallurgical condition of a machined surface. However, a common feature is the occurrence of a very hard and thin white layer at the surface which affects the properties of the material. The nature of the white layer formed on an EN. 24 steel by centreless grinding is described, and the effects of workpiece speed and grinding-wheel condition on the thickness of the white layer are presented and the results discussed in terms of Shaw's indentation model of grinding. Surface roughness is shown not to be a measure of the thickness of the white layer.