Light scattering has been used to characterise the periodic surface topography which develops on (001) Si epitaxial layers growing by a step flow mechanism. The periodicity provides a measure of the average kink density along the misorientation steps which develop a sawtooth form during growth. The diffracting properties of the layers have been determined after growth by angle-resolved scattering measurements. In-situ light scattering at fixed angle has been used to fol low the build-up of the periodic step arrays. The periodic structure is a metastable state of the surface maintained by the supersaturation during growth, and it decays on switching off the silane source gas or when the temperature is reduced below the point at which surface diffusion will support the sawtooth step shape.