In this Letter, the effect of CO2 contamination on nonaqueous Li-O2 battery rechargeability is explored. Although CO2 contamination was found to increase the cell's discharge capacity, it also spontaneously reacts with Li2O2 (the primary discharge product of a nonaqueous Li-O2 battery) to form Li2CO 3. CO2 evolution from Li2CO3 during battery charging was found to occur only at very high potentials (>4 V) compared to O2 evolution from Li2O2 (∼3-3.5 V), and as a result, the presence of CO2 during discharge dramatically reduced the voltaic efficiency of the discharge-charge cycle. These results emphasize the importance of not only completely removing CO2 from air fed to a Li-air battery, but also developing stable cathodes and electrolytes that will not decompose during battery operation to form carbonate deposits. © 2012 American Chemical Society.