A series of diagnostic experiments was undertaken in order to gain some understanding of the physical processes occurring in the high-power directly excited CO laser. These included studies of the UV and visible sidelight for a directly excited and flow-type CO laser; a comparison of various gas mixtures for laser operation at 77 and 195°K wall temperatures; a study of the laser spectral output as a function of cavity (wavelength-selective and free-running) operational mode (CW or Q-switched), gas pressure, and discharge current; and a measurement of the laser recovery time. Some of the more important conclusions of these experiments were that a CO-N2-O2-He mixture is superior to CO-N2-He or CO-O2-He mixture at 77 and 195°K; that freeze-out of O3and CO2is an important phenomenon at 77°K; and that both rotational and vibrational cross relaxation appear to be occurring in the CO during CW lasing. Also a discussion of three possible inversion mechanisms is included. An inversion based on the V-V cross-pumping theory of Treanor et al.  appears to be most consistent with our experimental results.