We have applied cavity ring-down spectroscopy to a kinetic study of the reaction of No-3 with CH3I in 20-200 Torr of N-2 diluent at 298 K. The rate constant of the reaction of NO3 + CH3I was determined to be (4.1 +/- 0.2) x 10(-13) cm(3) molecule(-1) s(-1) in 100 Torr of N-2 diluent at 298 K and is pressure-independent. This reaction may significantly contribute to the formation of reactive iodine compounds in the atmosphere.