The effect of spatial variability of the atmospheric CO2 distribution on temperature and water vapor retrievals from high spectral resolution observations of infrared emission is estimated by performing 4 different retrieval experiments. Using a global mean CO2 value, as currently routinely used, introduces errors in the retrieved temperature profile of up to 0.85 K compared to a retrieval in which the CO2 profile is known exactly. Including CO2 in the retrieval vector reduces these errors to 0.3 K. A more practical alternative, especially for data assimilation, is to use a monthly mean zonal mean CO2 value, which produces errors of up to 0.35 K in the temperature profile. ¿ 2001 American Geophysical Union