The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) curl molecule' s(-1). The UV spectrum of CH3CH2C(O)O-2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/-0.19) x 10(-18) cm(2) molecule(-1), k(CH3CH C(O)O, + CH3CH C(O)O-2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O-2 + CH3CH2O2) = (1.20 +/-0.06) x 10(-11) cm(3) molecule' s(-1), where quoted uncertainties only represent 2 sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O-2 with, and without, NOx were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.