The branching ratio beta = k(1b)/k(1a) for the formation of methyl nitrate, CH3ONO2, in the gas-phase CH3O2 + NO reaction, CH3O2 + NO -> CH3O + NO2 (la), CH3O2 + NO -> CH3ONO2 (1b), has been determined over the pressure and temperature ranges 50-500 Torr and 223-300 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K, the CH3ONO2 yield has been found to increase linearly with pressure from 0.33 +/- 0.16% at 50 Torr to 0.80 +/- 0.54% at 500 Torr (errors are 2 sigma). Decrease of temperature from 300 to 220 K leads to an increase of beta by a factor of about 3 in the 100-200 Torr range. These data correspond to a value of beta approximate to 1.0 +/- 0.7% over the pressure and temperature ranges of the whole troposphere. Atmospheric concentrations of CH3ONO2 roughly estimated using results of this work are in reasonable agreement with those observed in polluted environments and significantly higher compared with measurements in upper troposphere and lower stratosphere.