The rate constant for the unimolecular decomposition of the methoxy radical, CH3O + M → CH2O + H + M, is determined both theoretically and experimentally. In the theoretical calculations, potential energy surface information is obtained from ab initio multiconfiguration SCF and multireference configuration interaction calculations using basis sets of up to triple-ζ plus polarization quality. The zero point corrected forward and reverse barriers are calculated to be 25.6 and 8.0 kcal/mol, respectively. RRKM rate calculations are performed incorporating a quantum correction due to tunneling through an Eckart barrier fit to represent the MRCI/TZP energetics and the shape of the MC-SCF/DZP vibrationally adiabatic potential energy curve in the saddle point region. The calculated values compare closely with experimental data derived from kinetic modeling of CO formation rates measured in the thermal decomposition of methyl nitrite at 550-700 K in a static cell and at 1060-1620 K in shock waves.