Embedded cluster models have been used to model the molecular adsorption of methane and methyl at the perfect (100) surface of MgO. Energies are computed using the modified coupled pair functional method, with corrections for basis set superposition errors applied both during geometry optimization and to adsorption energies. In the case of methane, interadsorbate interactions are taken into account through ab initio pair-interaction energies, facilitating adsorption energies at monolayer coverage. Methane is found to adsorb preferentially at magnesium sites, in a dipod configuration, i.e. with two hydrogen atoms pointed down and towards oxide anions. At monolayer coverage, neighboring methane molecules are rotated 90 o relative to each other, keeping the dipod orientation. An adsorption energy of 8.5kJmol - 1 is obtained for methane, which is somewhat low compared to experimental estimates. Methyl adsorbs preferably over Mg 2 + sites, with a binding energy only slightly higher than for methane. No elements of covalency was detected in the bond between the radical and the MgO(100) surface.