Thermal equilibrium in planetary atmospheres occurs at altitudes where the ion, electron, and neutral temperatures are equal. Thermal equilibrium is postulated to occur in the collision-dominated lower ionosphere. This postulated altitude is above the lower boundary of all empirical models of planetary ionospheres. Physics-based model predictions of the altitude can not be validated due to a lack of adequate simultaneous observations of temperature profiles. This study presents temperature profiles from simultaneous observations on Atmosphere Explorer–C below 140 km and quiet-time neutral observations from TIMED/GUVI over Millstone Hill. These are compared with profiles from physics-based models with a discussion of their respective limitations. We conclude that there does not yet exist a quantitative understanding of the ion, electron, and neutral thermalization processes in low-altitude planetary ionospheres. Progress on this topic requires an adequate database of simultaneous ion, electron, and temperature profiles in the 110 to 140 km altitude range.