Thermodynamic properties of the methylmethoxy (CH3OCH2) radical relevant to the pyrolysis and combustion of dimethyl ether are presented from quantum chemical calculations. The potential energy surface with torsional coordinates of the methyl and methylene groups of CH3OCH2 is obtained at the CCSD(T)/aug-cc-pVTZ//B2PLYP/TZVP level. Internal rotations in the CH3OCH2 geometry are found to generate two structures which are nonsuperposable mirror images, and a "double-well" feature is observed on the one-dimensional potential of the methylene rotation. Using the resulting torsional potentials, multiple structure and torsional anharmonicitities of CH3OCH2 are evaluated by the multistructural method to obtain conformationally averaged partition functions which then serve as a basis for calculations of thermochemical parameters. The thermodynamic properties C-p degrees, S degrees, and H-T - H-0 at 298 K for CH3OCH2 are predicted to be 64.54 J K-1 mol(-1), 283.73 J K-1 and 14.34 kJ mol(-1), respectively. The computational method CCSD(T)/cc-pV(5,6)Z//B2PLYP/TZVP with isodesmic reactions determine Delta H-f degrees(298) (CH3OCH2) to be on average 1.97 +/- 0.64 kJ mol(-1) with anharmonic torsion included, and this value is predicted to be 1.72 +/- 0.62 kJ mol(-1) by the G4 method. Results from atomization energy calculations using the CCSD(T)/cc-pV(5,6)Z//B2PLYP/TZVP method yields an enthalpy of formation value of 4.14 kJ mol(-1). The isodesmic reaction scheme is found to give an enthalpy of formation more accurate than atomization energy approach.