An analysis is presented of ion temperature and concentration data from the retarding potential analyzer on board the Ogo 6 satellite. The investigation is restricted to plasma measurements in the undisturbed nighttime nonpolar ionosphere between 400 and 450 km spanning several months near solstice; under these conditions, observed ion temperatures are assumed to approximate the neutral gas temperature closely. Emphasis is placed upon statistical analyses of all data acquired within small cells of finite width in latitude and day of observation. Physical effects induced by changing solar and geophysical conditions are considered by comparing these average temperatures with the mass spectrometer and incoherent scatter (MSIS) atmospheric model. Measured nighttime temperatures between low summer and middle winter latitudes are, on the average, about 50 ¿K lower in magnitude than those calculated from the model. The MSIS model does, however, provide an excellent representation of the observed long-term temperature variations. Measured total concentrations of the molecular ions near the equator are larger in magnitude than those calculated according to equilibrium ion chemistry; this difference exceeds current stated uncertainties in either the reaction rates or the supporting neutral composition and suggests that important problems remain in our quantitative understanding of the ion chemistry of the nighttime F region. On the other hand, changes in the equatorial molecular ion concentrations are directly related to similar variations in the observed ion temperatures and in the MSIS neutral atmosphere.