Flash photolysis combined with UV time-resolved spectroscopy has been utilized to investigate the composite UV cross section for methyl formate peroxy (CH3OC(O)O-2 and O2CH2OC(O)H) between 210 and 320 nm. The self-reaction rate constant for methyl formate peroxy was measured at 298 K and found to be (2.3 +/- 0.7) X 10(-11) cm(3) molecule(-1) s(-1). Evidence is reported for the production of OH radical and CH2O from the decomposition of the activated methyl formate peroxy radical produced by the addition Of O-2 to methyl formate radical, and for the formation of ClO from the Cl + peroxy reaction. High level ab initio calculations were performed to explore the methyl formate oxidation mechanism as well as to assign the experimentally observed UV spectrum. The calculated UV spectrum band center is found to be within 10% of the experimentally determined methyl formate peroxy spectrum. High-resolution IR detection of the CH2O fragment was used to probe the branching ratio for unimolecular dissociation relative to collisional stabilization of the activated methyl formate peroxy radical as a function of pressure. The dissociation kinetics is described using a modified Lindemann mechanism.