Flash photolysis coupled to UV absorption spectrometry was used for a relative measurement of the (CH3)2CHCO and (CH3)3CCO radical decomposition rate constants. The reference reaction was the association reaction of (CH3)2CHCO and (CH3)3CCO with O2, for which the same rate constant as that of the CH3CO + O2 reaction was assumed: k2 = 3.2*10-12 cm3 molecule-1 s-1,25 independent of temperature. The rate expressions obtained at atmospheric pressure (N2 + O2) for decomposition are k((CH3)2CHCO) = 3.810(121)exp[-(5970600)K/T] s-1 (413 K < T < 503 K) and k((CH3)3CCO) = 1.6510(121)exp[-(4870310)K/T] s-1 (323 K < T < 453 K). RRKM calculations were also performed in order to assess the pressure dependence of rate constants. The reactions are in the falloff region at atmospheric pressure and in the temperature range investigated. Calculations were fitted to experimental data and have yielded at the high pressure limit k((CH3)2CHCO) = 3.31014exp(-7145K/T) s-1 and k((CH3)3CCO) = 1.11014exp(-6090K/T) s-1, in fairly good agreement with the expressions derived from Transition State Theory calculations. The detailed results are given in the form of the Troe expression. The atmospheric implications of the results are discussed. In particular, the results suggest that the class of RCO radicals containing only alkyl R groups cannot decompose in the troposphere and thus, add to O2 to form acylperoxy radicals RC(O)O2.