Single-pass gain has been measured for flowing CO2, CO2-N2, CO2-He, CO2-N2-He, and CO2-N2-H2mixes. The gain for CO2-N2mixes varies as d-0.9, where is the tube diameter. The diameter dependence of the gain is less pronounced for CO2- N2-He mixes; a peak gain of 4.7 dB/m was obtained in a 1/2 in diam tube. Fluorescence data indicate that the upper laser level population is saturated at 100 mA in all cases. The addition of He, H2, or O2depopulates the lower laser level; helium further increases the population of the upper laser level. The addition of CO increases the population of the upper laser level, probably by resonant transfer from the excited vibrational states of CO.