Sulfur hexafluoride was released from a small reservoir on the side of a sounding rocket equipped with a negative ion mass spectrometer. The effects of the release were observed only at altitudes between 100 km and the rocket's apogee at 128 km. Below these altitudes the SF6 was swept away by the flow of atmospheric gases past the rocket. The maximum ion signals were observved at 115-118 km, where the mean free path equaled the distance from the reservoir to the mass spectrometer. The negative ions that resulted from the SF6 release were SF6-, SF5-, F-, and O-. At 155 km the relative intensities of these ions were 1.0, 0.3, 0.07, and 0.06, respectively. A large fraction of the SF6- sampled by the mass spectrometer was probably in a mestastable state. The other observed ions were formed by dissociative electron attachment to SF6, collision-induced dissociation of SF6-*, and ion-molecule reactions. The mass spectrometer sampled a nonequilibrium distribution of ions in the first millisecond after they were formed. In addition, the large draw-in field of the mass spectrometer probably altered the identities and relative intensities of the ions. Laboratory measurements with a selected ion flow tube apparatus show that the reaction SF5-+O→F-+SF4O proceeds with rate constant 1.1¿10-11 cm3 s-1 at 300 K. ¿ American Geophysical Union 1987