The abundance ratio of O++/O+ has been observed to reach values of unity and above in the equatorial plasmasphere at L~3. This is two to three orders of magnitude greater than the relative O++ abundance at corresponding latitudes in the topside ionosphere. In this paper we pursue our earlier suggestion that thermal diffusion of O++, driven by the temperature gradient between the ionosphere and equatorial plasmasphere, is responsible. We have carried out this study by numerically integrating the time dependent diffusion equation for equinox conditions. Recent plasmaspheric data from the GEOS-1 spacecraft have been incorporated into a realistic model of the temperature an densities of major ion species extending from the ionosphere to the equatorial plane. Convection as well as local time variations and details of ion chemistry have been included in the model. Our main conclusion is that only thermal diffusion can account for observations of O++/O+~1 in the equatorial plasmasphere. Furthermore, we have found that there is a threshold temperature gradient between ionosphere and equatorial plasmasphere, corresponding to an equatorial temperature at L = 3 of ?5000 K, below which thermal diffusion becomes ineffective.