Laboratory and field studies have been carried out to determine the equilibrium constant for the system, N2O?NO5+NO2. In the laboratory studies a 170-m multiple-pass cell was used to study the equilibrium with the reactants at the ppm level in 700 torrs N2/O2 near 298 K. A Fourier transform infrared spectrometer was employed to follow N2O5, and an ultraviolet/visible differential optical absorption spectrometer to determine NO3 and NO2. The laboratory results for the Keq for temperature near 298 K are in good accord with the less direct estimates published by Graham and Johnston, but they are significantly different than some other recent direct measurements. Through measurements of 3>, 2>, 3>, and temperature in a 10.25-km path in the remote troposphere of Colorado, indirect determinations of the Keq have been made for the temperature range 269-281 K. The estimates of Keq at the lower temperatures of the atmosphere, although somewhat higher than expected from the extrapolation of published data from higher temperatures, are also in accord with the Graham and Johnston data within our large experimental error of the field measurements.