Photolysis of CO2 in the upper atmosphere of Mars results in escaping oxygen and residual CO. Proceeding at a rate of .5--3 ¿ 1010 mole yr-1, this process could reduce all of the atmospheric CO2 to CO in 20--140 Myr. The persistence of CO2 is maintained by re-oxidation of CO through several processes that duplicate the classical water-gas reaction, resulting in the destruction of water at a rate of a few mm per Myr. Owing to the continual decay of the solar EUV flux, oxygen escape and water loss have diminished with time roughly as t-2. If, as seems likely, a predominantly CO2 atmosphere appeared within 100 Myr of the formation of the planet, Mars should have lost several hundred meters of water over geologic time. Since very little carbon has escaped, the mass of the atmosphere has not been much greater than at present during the past 3.5 Gyr.