The rate constant of the reaction of Cl atoms with methacrolein (k(1)) has been measured relative to that of Cl with propane (k(2)) or cyclohexane (k(6)) at ambient temperature and pressures varying from 1-950 Torr. The experiments were carried out by irradiation (350 nm) of Cl-2/methacrolein/propane or cyclohexane mixtures in N-2 or N-2/O-2 diluent at ambient temperature in a spherical (500 cm(3)) Pyrex reactor (GC/FID analyses) or a 140 L FTIR smog chamber. The measured relative rate constant ratios in the GC/F1D experiments were k(1)/k(2) = 1.464 +/- 0.015(2 sigma) in N-2 and k(1)/k(2) = 1.68 +/- 0.03(2 sigma) in N-2/O-2 diluent (O-2 > 20 000 ppm). No pressure dependence was observed over the range studied in N2 (120-950 Torr) using the GC/FID. In the FTIR/smog chamber experiments values of k(1)/k(6) = 0.645 +/- 0.032, 0.626 +/- 0.037, 0.586 +/- 0.026, and 0.479 +/- 0.024 were measured in 700, 100, 10, and 1 Torr, respectively, of N-2 diluent. Using k(2) = (1.4 +/- 0.2) x 10-1 and k6 = (3.3 0.5) x 10-1 high pressure limiting rate constants of ki = (2.05 0.3) x 10-' [GC/FID] and (2.13 0.34) x 10-'0 [FTIR] cm3 mo1ecule-1 s-1 were determined. In experiments using the GC/FID reactor with NI, diluent the following products (molar yields) were observed: 2,3-dichloro-2methylpropanal [(47.2 8)% excluding error in calibration]; methacryloyl chloride [(22.9 2)%]; and 2-chloromethylacrolein [(2.3 0.8)%]. Addition of 200 ppm 02 (with CI, = 5000 ppm) resulted in a sharp reduction of the 2,3-dichloro-2-methylpropanal yield (to -2%) with an accompanying appearance of chloroacetone [yield = (55 7)% decreasing to (44 7)% in air diluent]. The methacryloyl chloride yield was 23% for [0214021 ratios from 0 to 0.2 but decreased to near zero as the 02/C12 ratio was increased to -400. The variation in methacryloyl chloride yield with the 02/Cl2 ratio in the initial mixture allowed an approximate measurement of the rate constant for the reaction of the methacryloyl radical with 02 relative to that with Cl2 (kolka, = 0.066 0.02). In experiments using the FTIR reactor in 700 Torr of N2 diluent, methacryloyl chloride [(26 3)%i and HC1 [(27 3)%] were observed as products. In 700 Torr of air diluent, the observed products were: chloroacetone [(44 5)%], CO2 [(27 3)%], HC1 [(2 I 3)%], and HCHO [(I4 2)%], and CH3C(0)CH2OH (3-4%). The observation of CH3C(0)CH2OH indicates the presence of OH radicals in the system. At atmospheric pressure and 297 K, the title reaction proceeds [(24.5 5)%] via abstraction of the aldehydic hydrogen atom, [(2.3 0.8)%] via abstraction from the -CH3 group, and approximately [(47 8) %] via addition to the C1-12=C< double bond with most of the addition occurring at the terminal carbon atom (uncertainties represent statistical 2o). The results are discussed with respect to the literature data.