The effects of the cathode materials platinum, molybdenum, copper and aluminium and the gas additives hydrogen and xenon are assessed for the construction of sealed off carbon monoxide lasers (5-6 mu m) operating at room temperature. The elimination of hydrogen impurity is shown to be essential for long life operation. The removal of hydrogen is effected by diffusion across a heated palladium tube. CO lasers have been operated at up to 10% efficiency. One sealed off laser with an initial gas fill of 1.3 Torr CO, 18.7 Torr He and 2 Torr Xe and a copper cathode has operated continuously for over 2500 h giving 14 W of laser power at 7% efficiency from an active length of 90 cm.