Experimental results are presented showing the effect of tube diameter on the output power and efficiency of CO2lasers. It is shown that both output power and efficiency are almost independent of diameter over the range 11mm to 44mm. The measured values of the output power and efficiency are 75 watts per meter and 10 percent. Higher efficiencies are obtained at lower power outputs. Mirror damage is discussed and two experiments involving the irradiation of CO2and ammonia with a high power laser beam are mentioned. An explanation is given for the strong interaction between the laser beam and the discharge tube current. Finally, the flow calorimeter used to measure high power laser beams is described.