In this paper we demonstrate that it is possible to invert twilight observations of the O+(2D-2P)-7320 ¿ airglow emission to obtain information about both the thermospheric atomic oxygen densities and the unattenuated O+(2P) ionization frequency. The efficacy of the proposed approach, which relies upon making twilight observations in more than one viewing direction, is illustrated using a synthetic data set and an inversion algorithm based on a simple photochemical model. The results of this study show that day-to-day variations in the thermospheric oxygen atom densities may be monitored from the ground without requiring complementary measurements of the solar EUV flux. The study also shows that twilight observations may be used to monitor variations in the solar flux components that are responsible for O+(2P) production and EUV heating of the upper thermosphere. ¿American Geophysical Union 1991