The gas-phase equilibrium of CH3ONO (cis) reversible arrow CH3ONO (trans) has been reinvestigated by FTIR spectroscopy in order to test the assumption that the equilibrium is not disturbed by the quenching process during formation of an Argon matrix. We are able to show that for this system this assumption is valid because the resulting thermodynamic parameters K-x(298) = 0.7 +/- 0.1, Delta H-R degrees = 3.5 +/- 0.2 kJ mol(-1), and Delta S-R degrees = 8.5 +/- 1 J mol(-1) K-1 for the matrix experiments are identical within experimental error limits to those obtained by other methods. In addition, for several infrared band pairs of both isomers, the relative extinction coefficients are determined.