The branching ratio beta = k(1b)/k(1a) for the formation of ethyl nitrate, C2H5ONO2, in the gas-phase C2H5O2 + NO reaction, C2H5O2 + NO -> C2H5O + NO2 (1a), C2H5O2 + NO -> C2H5ONO2 (1b), was determined over the pressure and temperature ranges 100-600 Torr and 223-298 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K the C2H5ONO2 yield was found to increase linearly with pressure from about 0.7% at 100 Torr to about 3% at 600 Torr. At each pressure, the branching ratio of C2H5ONO2 formation increases with the decrease of temperature. The following parametrization equation has been derived in the pressure and temperature ranges of the study: beta(P,T) (%) = (3.88 x 10(-3).p (Torr) + 0.365).(1 + 1500(1/T - 1/298)), The atmospheric implication of the results obtained is briefly discussed, in particular the impact of beta on the evolution of ethyl nitrate in urban plumes.