We present results of a comprehensive study of exospheric temperature T∞ at Millstone Hill. T∞ was calculated using zenith measurements of ion and electron temperature taken by the Millstone Hill incoherent scatter radar and a heat balance calculation during 201 experiments between 1981 and 1997. Fourier decomposition of the data is carried out to investigate how diurnal means and diurnal amplitudes and phases vary with season, solar, and magnetic activity and to search for a long-term trend in the diurnal mean. The results show generally good agreement with the mass spectrometer/incoherent scatter-86 model, and the data will be included in the next update to the model. It is found that the diurnal mean T∞ increases with increasing solar and magnetic activity and depends on season, with largest values in summer. No significant long-term trend is found in this data set. If there is saturation of T∞ at high levels of solar activity, it is not statistically significant in these data. The diurnal amplitude increases with increasing solar activity, but no other statistically significant dependencies are found. The diurnal phase has an earlier phase maximum in summer. Significant semidiurnal and terdiurnal components are found during some times but not others. ¿ 1998 American Geophysical Union