An electrochemical source for the reproducible generation and dosing of halogens in UHV systems is described. This approach minimizes corrosion damage to vacuum hardware and lends itself to the study of both submonolayer and multilayer halogen adsorption. The corrosive action of halogens on ultrahigh vacuum components has necessitated the development of molecular halogen beam sources, which allowinsitugeneration and dosing of fluorine, chlorine, bromine, and iodine, with a negligible increase in vacuum chamber background pressure. This communication describes the chlorine and bromine sources developed and routinely used in our laboratory. Iodine dosing may be accomplished using a source of similar design. Fluorine, however, because of its highly reactive nature, requires a somewhat different design of source, such as that recently described in detail by Bechtold.1