A new method for studying grain boundaries of polycrystalline CdSe semiconductors by the use of photoelectrochemical etching (photoetching) has been described. The selectivity of the photoetching site of CdSe depends on the grain structure, for instance on the presence of grain boundaries or on crystallographic orientation of the grain bulk. After the photoetching treatment of CdSe under weak anodic polarization, the grain boundaries are left undissolved. This finding makes it possible to observe directly the grain boundaries as separated from the grain bulk. A transmission electron micrograph and a selected area diffraction pattern of the grain boundary are shown.