[en] For tokamak wall conditioning, surface cleaning of molybdenum by chemical reaction with oxygen and hydrogen gases has been studied experimentally by Auger electron spectroscopy (AES) and Auger/Sputter techniques. Means of pretreatments, buffering with water containing Al₂O₃ powder, glow discharge cleaning, and electropolishing show similar results. By exposing molybdenum sample to 0.1 Torr oxygen and 1 Torr hydrogen successively at 500⁰C, the impurity content on the surface reduces to 1/3 -- 1/4 of the initial value. When the content is reduced below the detectable limit of AES by heating up to 1,800⁰C in vacuum, it becomes 1/6 -- 1/7. By heating at 500⁰C, the impurities in the surface penetrate inward, probably because of the increase of impurity diffusion at this temperature. (auth.)