The rate coefficient for the gas-phase reaction of chlorine atoms with dimethyl malonate (DMM, CH3OC(O)CH2C(O)OCH3) was determined at 298 K using relative methods giving a value of (3.8 +/- 0.4) x 10(-12), cm(3) molecule(-1) s(-1)). The photo-oxidation mechanism of DMM was also investigated. The main products were identified by infrared spectroscopy, and computational calculations were performed in order to support the experimental data. The results reveal that the photo -oxidation occurs mainly by the abstraction of an H atom from the methyl groups. The CH3OC(O)CH2C(O)OCH2O center dot radical formed subsequently reacts according to three competitive paths: reaction with molecular oxygen to yield CH3OC(O)CH2C(O)OC(O)H, isomerization-unimolecular decomposition to lead finally to CH3OC(O)C(O)H, CO2, and HC(O)OH, and alpha-ester rearrangement to form monomethyl malonate and carbon monoxide. The yield of products as a function of oxygen pressure was also determined.