Model calculations, as well as optical observations, have shown that hot oxygen is the dominant neutral constituent up to about 3000 km in the upper atmosphere of Venus. Consequently O+ is expected to be the dominant ion up to about 3000 km in the upper ionosphere of Venus. Under normal circumstances, it is difficult to verify this result, since the solar wind interaction terminates the Venus ionosphere at altitudes below about 500 km for subsolar conditions. However, the upper boundary of the Venus ionosphere can move up to very high altitudes during some rare events when the solar wind dynamic pressure is extremely low. An analysis of Pioneer Venus measurements during such events indeed shows O+ to be the dominant ion at altitudes even as high as 2800 km. This result provides ionospheric evidence of hot oxygen in the upper atmosphere of Venus. ¿ American Geophysical Union 1992