The kinetics of the gas phase reactions of the Criegee intermediate CH2OO with O-3 and IO have been studied at 296 K and 300 Torr through simultaneous measurements of CH2OO, the CH2OO precursor (CH2I2), O-3, and IO using flash photolysis of CH2I2/O-2/O-3/N-2 mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Experiments were performed under pseudo-first-order conditions with respect to O-3, with the rate coefficients for reactions of CH2OO with O-3 and IO obtained by fitting to the observed decays of CH2OO using a model constrained to the measured concentrations of IO. Fits were performed globally, with the ratio between the initial concentration of O-3 and the average concentration of IO varying in the range 30-700, and gave k(CH)(2OO+)(O3)( )= (3.6 +/- 0.8) x 10(-13) cm(3) molecule(-1) s(-1) and k(CH)(2OO+)(IO) = (7.6 +/- 1.4) x 10(-11) cm(3) molecule(-1) s(-1) (where the errors are at the 2 sigma level). The magnitude of k(CH)(2OO+O3) has a significant effect on the steady state concentration of CH2OO in chamber studies. Atmospheric implications of the results are discussed.