The kinetics and mechanism of the reactions Cl + (CH3)(3)CCHO and Cl + (CH3)(2)CHCHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/Fourier transform infrared (FTIR) smog chamber. Reactions proceed predominantly by abstraction of the aldehydic H atom to form acyl radicals. FTIR measurements indicated that the acyl-forming channel accounts for 81%+/-8% and 85%+/-10% of the reaction of Cl atoms with (CH3)(3)CCHO and (CH3)(2)CHCHO. UV measurements indicated that the acyl-forming channel accounts for 88%+/-6% and 85%+/-5% of the reaction of Cl atoms with (CH3)(3)CCHO and (CH3)(2)CHCHO. The atmospheric fate (>98%) of the resulting (CH3)(3)CC(O) and (CH3)(2)CHC(O) radicals is an addition of O-2 to give the corresponding acylperoxy radical. In 700 Torr of N-2/O-2 mixtures at a temperature of 296 K, the decomposition of (CH3)(3)CC(O) and (CH3)(2)CHC(O) radicals via CO elimination occurs at rates of similar to10 X 10(5) and similar to4 x 10(3) s(-1), respectively. Relative rate methods were used to measure the reaction rates (in units of cm(3) molecules(-1)): k(Cl + (CH3)(3)CCHO) = (1.15+/-0.30) x 10(-10) k(Cl + (CH3)(2)CHCHO) = (1.33+/-0.25) x 10(-10); k(Cl + (CH3)(3)CC(O)Cl) = (6.86+/-1.50) x 10(-12); k(Cl + (CH3)(2)CHC(O)Cl (7.82+/-2.10) x 10(-12); k(Cl + (CH3)(3)CCl) = (1.27+/-0.21) x 10(-11); and k(Cl+ (CH3)(2)CHCCl = (2.01+/-0.49) x 10(-11). Self-reaction rate constants were measured for (CH3)(3)CC(O)O-2 and (CH3)(2)CHC(O)O-2 radicals and compared to previous measurements.