The rate constant for the reaction of di-tert-butyl malonate (DTBM) with chlorine atoms in the gas phase was measured using cyclohexane and pentane as references. The measurements lead to a value of (1.5 +/- 0.1) x 10(-10) cm(3) molecule(-1) s(-1). The subsequent photo -oxidation mechanism of DTBM in the absence and presence of nitrogen dioxide was investigated. The main carbonated products identified in the first case were acetone, formic acid, carbon monoxide, and carbon dioxide. The addition of nitrogen dioxide lead besides to the formation of (CH3)(3)CC(O)OONO2 and (CH3)(3)CONO2. The proposed photo -oxidation mechanism was supported both experimentally and computationally. The results reveal that the (CH3)(3)COC(O)CH2C(O)OC(CH3)(2)O-center dot radical formed reacts according to two competitive reactions: decomposition to yield acetone and (CH3)(3)COC(O)CH2C(O)O-center dot radical 55 +/- and migration of the H atom of the methylene group to the terminal oxygen atom 40 +/- 3%.