The chemistry of the acetonoxy radical, CH3C(O)CH2O. formed in the atmospheric degradation of acetone, was studied dy a combination of experimental and theoretical methods. In an environmental chamber study conducted over the temperature range 225-298 K, acetonoxy radical chemistry was shown to be dominated by decomposition, CH3C(O)CH2O -> CH3C(O) + CH2O. No evidence was found for a reaction of this species with O-2, CH3C(O)CH2O + O-2 -> CH3C(O)CHO + HO2, even at 225 K in the presence of 1 atm O-2. In a theoretical ab initio and statistical kinetics investigation, the barrier to CH3C(O)CH2O decomposition was found to be 6-7 kcal/mol. Using SSE theory and RRKM-based master equation analysis, it was determined that about 80% of the CH3C(O)CH2O radicals formed in the CH3C(O)CH2O2 + NO reaction have sufficient energy to decompose "promptly" under tropospheric conditions. On the basis of TST theory and allowing for falloff, the dissociation rate of thermalized CH3C(O)CH2O radicals was found to be on the order of 5 x 10(7) s(-1) at 1 atm and 300 K and 5 x 10(5) s(-1) at 0.2 atm and 220 K. The results confirm that the acetonoxy reaction with O-2 is always negligible in the troposphere, consistent with the experimental observations. As part of this study, the rate coefficient for reaction of CI with acetone (k(2)) was measured by a relative rate technique, and a value of k(2) = (3.1 +/- 0.5) x 10(-11) exp(-815 +/- 150/T) cm(3) molecule(-1) s(-1) reported.