Adsorptions of hydrogen and methane on  well-outgassed magnesium oxide surfaces were investigated at room temperature using mainly a temperature-programmed desorption technique. These adsorbates behave as Bronsted acid on the nearest pair of lowly-coordinated magnesium and oxygen ions, and are finally adsorbed with heterolytic dissociation. Under severer adsorption conditions carried out at high temperatures or under ultraviolet irradiation, lowly-coordinated magnesium and oxygen ions, which are located separately each other, can also participate in the adsorption. In case of hydrogen, however, adsorption under these severer conditions leads to the most stable homolytic dissociation, while in case of methane only heterolytically dissociated adsorption proceeds.These experimental results have been well explained on the basis of ab initio molecular orbital calculations.