[en] This report discusses frequency tripling of CO₂ lasers in deuterium chloride (DCl), deuterium bromide (DBr), and selenium (Se) gases. Theoretical tripling equations are reviewed and summarized. Third order nonlinear susceptibilities, chi/sup (3)/, for DCl, DBr and Se are calculated. Experimental measurements of frequency tripling in high pressure DCl and DBr are discussed. Resonantly enhanced four-wave sum-frequency generation in Se using simultaneous 9.5 and 10.6 μm TEA CO₂ laser pulses is demonstrated. The necessary selenium vapor density is generated in a shock tube. All of the experiments are in relatively good agreement with theory and results in chi /sup (3)/ values of 1.4 x 10^-37 ESU for DBr, 2.0 x 10^-36 for DCl and 1.1 x 10^-33 for Se (at 2000⁰K). Scaling equations are discussed which predict frequency conversion efficiencies of > 50% in DCl and Se using gigawatt power, nanosecond pulsed CO₂ lasers and reasonably sized conversion cells