Smog chamber FTIR techniques were used to study the atmospheric oxidation of methyl propionate in 740 Torr of air in the presence of NOx at 296 +/- 2K. Relative rate techniques were used to measure k(OH + CH3CH2C(O)OCH3) = (9.29 +/- 1.13) x 10(-13), k(Cl + CH3CH2C(O)OCH3) = (1.51 +/- 0.22) x 10(-11), k(Cl + CH3CH2C(O)OC(O)H) = (2.89 +/- 0.35) x 10(-12), k(Cl + CH3CH2C(O)OH) = (4.72 +/- 0.62) x 10(-12), and k(Cl + CH3C(O)C(O)OCH3) = (4.99 +/- 0.96) x 10(-13) cm(3) molecule(-1) s(-1). The products (and molar yields) formed in the Cl-atom initiated oxidation of methyl propionate were as follows: propionic formic anhydride (CH3CH2C(O)OC(O)H), 0.099 +/- 0.019; propionic acid (CH3CH2C(O)OH), 0.139 +/- 0.027; carbon monoxide, 0.132 +/- 0.026; methyl pyruvate (CH3C(O)C(O)OCH3), 0.289 +/- 0.057; acetaldehyde, 0.077 +/- 0.015; methoxy formylperoxynitrate (CH3OC(O)O2NO2), 0.083 +/- 0.016; methyl glyoxylate (H(O)CC(O)OCH3), 0.111 +/- 0.022; organic nitrates, 0.07 +/- 0.02; and formaldehyde. These products account for 79 +/- 16% of the loss of methyl propionate. The atmospheric oxidation mechanism of methyl propionate is presented and discussed.