We report a general technique for determining the gas phase equilibrium constant. K-p. of the A <----> 2C system using cavity ring-down spectroscopy (CRDS). Working at a constant temperature, the absorption of one of the equilibrium species is measured at two different total pressures. K-p is determined from the total pressures of the equilibrium mixture and the ratio of the absorptions. Theoretical limits on sensitivity of this technique are described as a function of experimental conditions. We present results from the reaction N2O4 <----> 2NO(2), measured at T = 283 K. K-p measured in this experiment, 21 (+/-5) Torr. is found to he lower than that recommended by the NASA Panel for Data Evaluation, 32 Torr. [S.P. Sander. A.R. Ravishankara. D.M. Golden. C.E. Kolb. M.J Kurylo, R.E. Huie, V.L. Orkin, M.J. Molina, G.K. Moortgat. B.J. Finlayson-Pitts. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies. Evaluation No. 14, Jet Propulsion Laboratory, Pasadena, CA, 2003]. (C) 2004 Elsevier B.V. All rights reserved.