The operation and performance of small-orifice hollow cathodes for use in electron bombardment ion thrusters is described. Using mercury propellant, vaporized either in a small boiler or by a heated porous plug, cathodes have been operated in both a simple test facility and in an ion thruster. Voltage-current characteristics were obtained by a novel rapid-scan technique, and the dependence of these on parameters such as vapor flow rate and temperature has been studied. An examination of such characteristics has enabled a qualitative theory of the cathode discharge to be proposed, which is consistent with the observed behavior. Cathodes have been operated for periods exceeding 1000 hr without any apparent deterioration of performance.