The branching ratio β = 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: β(P,T) (%) = (3.88 × 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 β on the evolution of ethyl nitrate in urban plumes.