The branching ratio β = k_1b/k_1a for the formation of iso-propyl nitrate, i-C₃H₇ONO₂, in the gas-phase reaction of i-C₃H₇O₂ with NO, i-C₃H₇O₂ + NO → i-C₃H₇O + NO₂ (1a), i-C₃H₇O₂ + NO → i-C₃H₇ONO₂ (1b), was determined over the pressure range 55-500 Torr at 298 K using a high-pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass-spectrometer. The β coefficient was found to increase linearly with pressure from about 0.6% at 55 Torr to about 3% at 500 Torr. The atmospheric implication of the results obtained is briefly discussed.