The rate of self-reaction of the simplest Criegee intermediate, CH₂OO, is of importance in many current laboratory experiments where CH₂OO concentrations are high, such as flash photolysis and alkene ozonolysis. Using laser flash photolysis while simultaneously probing both CH₂OO and I atom by direct absorption, we can accurately determine absolute CH₂OO concentrations as well as the UV absorption cross section of CH₂OO at our probe wavelength (λ = 375 nm), which is in agreement with a recently published value. Knowing absolute concentrations we can accurately measure k_self = 6.0 ± 2.1 × 10^–11cm³ molecule^–1 s^–1 at 297 K. We are also able to put an upper bound on the rate coefficient for CH₂OO + I of 1.0 × 10^–11 cm³ molecule^–1 s^–1. Both of these rate coefficients are at least a factor of 5 smaller than other recent measurements of the same reactions.