On the basis of the present status of positive ion chemistry, reliable laboratory measurements of reaction rates, and in situ meaurements of electron density and positive ion composition an attempt has been made to estimate the atmospheric temperature at the mesopause region for different geophysical conditions. It is found that at high-latitude mesopause, when the ionosphere is quiet, the temperature is about 130 K during summer and about 200 K during spring, consistent with the Cira (1972) model. During winter anomaly, a temperature larger by about 70 K than that of the normal level is estimated, also in agreement with the observation. But at mid-latitude during quiet conditions a temperature unexpectedly higher by about 60 K than its normal value is obtained from the ion composition data of August 12, 1976, at Wallops Island.