The title reaction was studied at 296 K and 0.38-940 Torr total pressure using a FTIR smog chamber technique. The overall rate constant for reaction of CH3OCH2 radicals with O-2 may be written, k(1) = k(RO2) + k(prod), where k(RO2) is the rate constant for peroxy radical production and k(prod) is the rate constant for the production of other species from reaction 1. k(1) was measured relative to the pressure independent reaction of CH3OCH2 radidals with Cl-2 (k(4)) There was no discernible effect of pressure on k(1) in the range 200-940 Torr. Between 200 and 2 Torr total pressure k(1) decreased by approximately a factor of 2. For pressures below 2 Torr k(1) was again independent of pressure. The reaction proceeds via the formation of an activated complex, CH3OCH2O2#, that is either collisionally stabilized to form the peroxy radical, CH3OCH2O2, or undergoes intramolecular H-atom abstraction followed by decomposition to give two formaldehyde molecules and an OH radical : CH3OCH2 + O-2 reversible arrow CH3OCH2O2#, CH3OCH2O2# + M --> CH3OCH2O2 + M, CH3OCH2O2# --> CH2OCH2O2H# --> 2HCHO + OH. The products from this reaction were studied as a function of total pressure. The molar yield of formaldehyde increased from <2% at 700 Torr total pressure to similar to 200% at 0.38 Torr total pressure, while the combined yield of methyl formate and methoxy methylhydroperoxide decreased from similar to 100% to 4% over the same pressure range. Fitting the product yields and relative rate data using a modified Lindemann expression gave the following rate constants : k(RO2.0)/k(4) = (1.97 +/- 0.28) x 10(-19) cm(3) molecule(-1), k(RO2 infinity)/k(4) = 0.108 +/- 0.004, and k(prod,0)/k(4) = (6.3 +/- 0.5) x 10(-2) where k(RO2.0) and k(RO2,infinity) are the overall termolecular and bimolecular rate constants for formation of the CH3OCH2O2 radical and k(prod,0) represents the bimolecular rate constant for the reaction of CH3OCH2 radicals with O-2 to yield formaldehyde in the limit of low pressure. These data and absolute rate data from the literature were used to derive a rate constant for the reaction of CH3OCH2 radicals with Cl-2 of (1.00 +/- 0.16) x 10(-10) cm(3) molecule(-1) s(-1). The results are discussed in the context of the use of dimethyl ether as an alternative diesel fuel.