Ozone-alkene reactions form vibrationally excited Criegee intermediates (of the form R1R2COO), some of which, once thermalized, are thought to react with SO2, H2O, NOx, aldehydes, and alcohols. Several studies using relative rate techniques or ab initio calculations have resulted in estimates for the rate coefficients of reactions of the thermalized biradicals. The ranges of measured and estimated rate coefficients span 2-6 orders of magnitude, depending on the reaction partner. Using an atmospheric pressure flow reactor, we have made the first absolute rate coefficient determinations for reactions of a thermalized Criegee intermediate, measuring rates for unimolecular decomposition and reaction with acetaldehyde. For the thermalized CH3-CHOO formed in trans-2-butene ozonolysis, values for k(dec) = (7)over dot 6 s(-1) and k(ald) = 1.0 x 10(-12) cm(3) molecule(-1) s(-1), accurate to within a factor of 3 and 6, respectively, were obtained.