The rate coefficient of a chemical reaction is given by the equation k=F∞0&sgr;vf (v) dv, where &sgr; is an energy dependent cross section, v is the relative speed of the reactants, and f (v) is the relative speed distribution. For certain highly energy dependent ionospheric reactions, k is controlled by the population in the high-energy tail of the relative speed distribution. This leads to problems in the application of laboratory measurements of rate coefficients to the atmosphere where f (v) can be significantly different for the same effective temperature. However, we show that the rate coefficients in the laboratory and in the thermosphere are approximately the same for the same value of the mean energy parameter KEcm if a thermal equilibrium apparatus such as a static or flowing afterglow system is used in the laboratory. The rate coefficients are in general not the same in the atmosphere as in drift tube experiments, although for O+ in a helium buffer the difference is about 25% or less. As an application of the present work we derive new expressions for the rate coefficients of O+ reacting with N2, O2, and NO in the ionosphere, using cross sections published by laboratory workers.