Carbon dioxide in the pressure range of 50–600 Torr has been photolyzed at 1470 Å. An initial quantum yield of Φ(CO) = 1.0 was observed. The O2/CO ratio was 0.35 ± 0.02. While the O2/CO ratio remained constant, the rate of CO formation decreased significantly with an increase in the photolysis time. Such a lowering of the conversion rate is attributed to a light attenuation due to solid deposit being formed on the lamp window.