Abstract: "The reflections of microwaves from the walls of a plasma-containing vessel are studied. The probable cause of the reflections is discussed, and several suggestions are made for eliminating the reflections from the waveguide in the vessel and particularly from the pyrex vessel wall and copper tubing surrounding the pyrex. Quarter-wave-thick carbon-filled plastics or glasses having a very low vapor pressure room most promising when placed inside the pyrex cylinder. BaTiO3 appears to be an excellent material when placed outside the vacuum."