Single crystals of CdS possess a translucent yellow color but when indium is diffused into the crystal from the vapor in an evacuated silica capsule it becomes opaque. This type of diffusion is linearly dependent on the indium concentration and produces a sharp interface between the diffused and undiffused regions of the crystal. The process of diffusion doping can be demonstrated by studying the variation of the depth of the interface with anneal time and temperature, using an optical microscope. In addition, as CdS possesses a hexagonal crystal structure of the ‘‘wurtzite’’ type, this is a sensitive method for studying diffusion anisotropy along the two principal crystal directions, and the sharpness of the interface can be demonstrated by the dispersion produced by a narrow beam of white light. This article describes how these properties can be incorporated into an undergraduate student experiment, or an open ended project for demonstrating Fick’s laws and the process of diffusion doping.