The temperature dependence of the collisional removal of O2 molecules in the &ugr;=9 level of the A3&Sgr;u+ electronic state has been studied for the colliders O2 and N2, over the temperature range 150 to 300 K. In a cooled flow cell, the output of a pulsed dye laser excites the O2 to the &ugr;=9 level of the A3&Sgr;u+ state, and the output of a time-delayed second laser monitors the temporal evolution of this level via a resonance-enhanced ionization. We find the thermally averaged removal cross section for O2 collisions is constant (~10 ¿2) between room temperature and 200 K, then increases rapidly with decreasing temperature, doubling by 150 K. In contrast, the N2 cross section at 225 K is ~8% smaller and gradually increases to a value at 150 K that is ~60% larger than the room temperature value. The difference between the temperature dependence of the O2 and N2 collision cross section implies that the removal by oxygen becomes more important at the lower temperatures found in the mesosphere, but removal by N2 still dominates.¿ 1997 American Geophysical Union