Using approximately 400 simultaneous measurements of ion and neutral densities and temperatures, and the spectrum of the solar flux measured by the Atmosphere Explorer C satellite, we have determined the rate constant k1 for the reaction between N2+ and O in the ionosphere for ion temperatures between N2+ and O in the ionosphere for ion temperatures between 600¿ and 700¿K. We find that k1=1.1¿10-10 cm3 s-1, with a standard deviation of ¿15%. If we use the temperature dependence for this reaction determined in the laboratory, namely, k1∝ (300/Ti)0.44, then at 300¿K we find that k1=1.4¿10-10 cm3 s-1, in excellent agreement with the recommended laboratory value.