At high temperatures carbon dioxide may absorb solar radiation and react to form carbon monoxide and molecular oxygen. The CO, so produced, may be converted by well-established means to a combustible fuel, such as methanol. We intend to make a future demonstration of the solar reduction of CO 2 based on these processes. This paper, however, addresses only the problem of preserving, or even enhancing, the initial photolytic CO by quenching the hot gas with colder H 2O or CO 2. We present model calculations with a reaction mechanism used extensively in other calculations. If a CO 2 gas stream is heated and photolyzed by intense solar radiation and then allowed to cool slowly, it will react back to the initial CO 2 by a series of elementary chemical reactions. The back reaction to CO 2 can be terminated with the rapid addition of CO 2, water, or a mixture. Calculations show that a three-fold quench with pure CO 2 will stop the reactions and preserve over 90% of the initial photolytic CO. We find that water has one of two effects. It can either increase the CO level, or it can catalyze the recombination of O and CO to CO 2. The gas temperature is the determining factor. If the quench gas is not sufficient to keep the temperature below approximately 1100 K, a chain-branching reaction dominates and the reaction to CO 2 occurs. If the temperature stays below that level a chain terminating reaction dominates and the CO is increased. The former case occurs below approximately a fourfold quench with a water/CO 2 mixture. The later case occurs when the quench is greater than fourfold. We conclude that CO 2, H 2O, or a mixture may quench the hot gas stream photolyzed by solar radiation and preserve the photolytic CO.