The rate constants of 10 radical formation from the reaction of CH2I with O-2 were determined in the pressure range of 5-80 Torr with N-2 diluent at 278-313 K, using cavity ring-down spectroscopy. The room temperature rate constant is (4.0 +/- 0.4) x 10(-13) cm(3) molecule(-1)s(-1) at 30 Torr total pressure. No significant dependences on temperature and total pressure were observed. The yield of 10 from CH2I + O-2 was estimated to be unity in 100 Torr total pressure of N-2 diluent.