Carbon dioxide number densities were measured for the first time in the sunlit lower thermosphere on June 29, 1974, above Wallops Island (38¿N). The CO2 concentrations were obtained with a mass spectrometer that employed a cryo-cooled ion source. Below the turbopause the CO2 densities correspond to the ground level mixing ration; above the turbopause the observed n (CO2)/n (Ar) ratio decreases more rapidly with altitude than is expected for diffusive equilibrium. Photolysis and reactions with the O+ ion alone remove sufficient amounts of CO2 to explain the observed deviation from a pure diffusive equilibrium separation.