We describe an analysis of processes affecting the vertical distributions and flows of deuterium and hydrogen between 100 and 500 km in the daytime thermosphere. A steady state numerical model includes HD and H2 molecular flows, relevant chemistry, and transport by both molecular and eddy diffusion. The magnitude and height variation of the vertical fluxes are determined that satisfy preestablished conditions on concentrations at the two boundaries. The results confirm the suggestion of Breig et al. (1987) that the outward thermospheric fluxes and hence the planetary loss rates for deuterium must be large compared to evaporative escape, as is also the case for hydrogen. Using the observational hydrogen data in the mesosphere and lower thermosphere and AE-C measurements of hydrogen and deuterium in the thermosphere, we derive nominal upward fluxes through the daytime thermosphere that exceed the corresponding Jeans evaporative escape rate by factors of ~5 for H and in excess of 500 for D. We have examined the dependence of these results on several pertinent parameters; while the flows are especially sensitive to the total abundance of each species and the molecular diffusion rates in the lower thermosphere, no reasonable changes were found sufficient to reduce the outward thermospheric flux of either D or H to the level associated with evaporative escape. We must thus conclude that large vertical flows of both deuterium and hydrogen in the thermosphere and comparably large planetary losses are required to provide consistency between current theory and the available observational data. ¿American Geophysical Union 1991