The temperature dependence of the NO2 + NO3 + (M) <-> N2O5 + (M) equilibrium has been studied in the temperature range 279.6-294.2 K at a total pressure of 1000 mbar synthetic air in the 200 m(3) EUPHORE simulation chamber facility in Valencia, Spain. Simultaneous determination of the concentration-time behaviors of NO3, NO2, and N2O5 were obtained using the DOAS technique for detection of the NO3 radical and FT-IR spectroscopy for detection of NO2, and N2O5. The experimental data are well described by ln([N2O5]/[NO2][NO3]) = -60.46 +/-0.26 + (10724 +/- 75)/T where the errors given are random and expressed as 95% confidence limits. A critical comparison, is made between the values of K-eq determined in this work and those reported in the literature.