Ethyl propionate is a model for fatty acid ethyl esters used as first-generation biodiesel. The atmospheric chemistry of ethyl propionate was investigated at 980 mbar total pressure. Relative rate measurements in 980 mbar N-2 at 293 +/- 0.5 K were used to determine rate constants of k(C2H5C(O)OC2H5 + Cl) = (3.11 +/- 0.35) x 10(-11), k(CH3CHClC(O)OC2H5 + Cl) = (7.43 +/- 0.83) X 10(-12), and k(C2H5C(O)OC2H5 + OH) = (2.14 +/- 0.21) X 10(-12) cm(3) molecule(-1) s(-1). At 273-313 K, a negative Arrhenius activation energy of -3 kJ mol(-1) is observed.. The chlorine atom-initiated oxidation of ethyl propionate in 980 mbar N-2 gave the following products (stoichiometric yields): ClCH2CH2C(O)OC2H5 (0.204 0.031), CH3CHClC(O)OC2H5 (0.251 +/- 0.040), and C2H5C(O)OCHClCH3 (0.481 +/- 0.088). The chlorine atom-initiated oxidation of ethyl propionate in 980 mbar of N-2/O-2 (with and without NOx) gave the following products: ethyl pyruvate (CH3C(O)C(O)OC2H5), propionic acid (C2H5C(O)OH), formaldehyde (HCHO), and, in the presence of NOx PAN (CH3C(O)OONO2). The lack of acetaldehyde as a product suggests that the CH3CH(O)C(O)OC2H5 radical favors isomerization over decomposition. From the observed product yields, we conclude that H-abstraction by chlorine atoms from ethyl propionate occurs 20.4 +/- 3.1%, 25.1 +/- 4.096, and 48.1 +/- 8.8% from the CH3-, CH2-, and -OCH2- groups, respectively. The rate constant and branching ratios for the reaction between ethyl propionate and the OH radical were investigated theoretically using quantum mechanical calculations and transition state theory. The stationary points along the reaction path were optimized using the CCSD(T)-F12/VDZ-F12//BH&HLYP/aug-cc-pVTZ level of theory; this model showed that OH radicals abstract hydrogen atoms primarily from the -OCH2- group (80%).