We have applied cavity ring-down spectroscopy to a kinetic study of the reaction of NO3 with CH2I2 in 25-100 Torr of N-2 diluent at 298 K. The rate constant of reaction of NO3 + CH2I2 is determined to be (4.0 +/-1.2) x 10(-13) cm(3) molecule(-1) s(-1) in 100 Torr of N-2 diluent at 298 K. The rate constant increases with increasing pressure of buffer gas below 100 Torr. The reaction of CH2I2 with NO3 has the potential importance at nighttime in the atmosphere. (c) 2006 Elsevier B.V. All rights reserved.