A description is given of a simple and efficient apparatus for purifying radon, which has given several years effective service in connection with radium therapy. Potassium hydroxide is used to remove carbon dioxide and most of the water. Since phosphorus pentoxide for drying has been dispensed with, only one Toepler pump and fewer valves and stopcocks are necessary. The number of stopcocks has been reduced to three and there is but one large bulb and one float valve. The impurities are removed by sparking and allowing excess hydrogen to escape through a palladium tube heated electrically; thus there is no danger of clogging the tap with particles of loose reagent. All parts are made of Gundlach glass. Purification takes from two to five minutes.