Reactions which lead to the solution or volatilization of molybdenum and which are suitable for high-precision etching of patterns in thin films of molybdenum are not well known. In a search for a process for etching cavities in films 1mu thick it has been found that the formation and simultaneous removal of molybdenum trioxide can be achieved by exposure of heated molybdenum films to a mixture of oxygen and hydrogen chloride gases. Etch rates of 1000 to 10,000 A/min have been obtained with specimen temperatures of 400 to 600C and with gas pressures between 0.1 and 1 torr. An etch factor of about 2 has been typical, and the final etched surface has been as smooth if not smoother than the original surface. Aluminum oxide of about 0.1mu thickness has been used as a resist. It is hoped than an electron-exposed organic resist can eventually be adapted for use with this process. Discussion and speculation on the mechanism of formation of the oxide and its removal have been included, as well as discussion of some of the factors which are important in predicting the optimum conditions for etching. A residue of a very thin, adherent layer of MoO2 on partially etched molybdenum surfaces has been a problem, and means for removing this film are discussed. The process appears to have promise for etching with dimensional control accurate to better than 0.1mu in films 1mu thick. Greater precision may be possible in films thinner than 1mu.