The reaction of dimethyl ether (1) with atomic oxygen generated by photolysis of ozone or N2O was examined in low-temperature matrices. The major reaction products are two conformers of methoxymethanol (5). IR absorptions of the products were assigned by isotopic labeling (O-18 and D) and DFT calculations at the B3LYP/ 6-311++G(d,p) level of theory. The mechanism of the formation of 5, in particular H abstraction from 1 by atomic oxygen ((OP)-P-3 and (OD)-D-1), was investigated using UMP, UCCSD(T), and UDFT. In both the H abstraction and the O(D-1) insertion reaction, the out-of-plane C-H bonds of 1 are preferentially attacked since the inplane C-H bonds are about 10 kcal/mol stronger. In the case of a reaction with O(P-3), an Arrhenius activation energy of 3.5 kcal/mol is calculated at 298 K, which compares well with an experimental value of 2.85 kcal/mol. In the exit channel of the reaction, a radical-radical complex between CH3CH2. and . OH (-2.7 kcal/mol relative to separated products) is found. The latter is the starting point for the formation of 5 and helps to rationalize the stereoselectivity of the reaction leading to particular conformations of 5.