An embedding method designed to provide a molecular level understanding Of adsorbate energetics and surface reaction mechanisms is presented. Electronic structures; including relativistic effects, are described by an ab initio formalism that permits an accurate determination of energies and adsorbate structure. Applications to catalytic and photochemical reactions on transition metal surfaces are reported. (I) In the catalytic conversion of ethyl adsorbed on platinum surfaces to ethylene, the minimum energy pathway for the transfer of hydrogen to the metal surface is calculated. A detailed description of the transition state is presented. (II) In the photodissociation of methane physisorbed on platinum, results are reported for a methane molecule interacting with an embedded cluster model of Pt(111). Configuration interaction theory is: used to-sort out states resulting from electron attachment to methane from lower energy states that correspond to metal. excitations. Energies of the ground and excited states of the Pt(111)CH4 complex are calculated as a function of geometrical distortions and the distance of methane from the-surface. Key steps in the photodissociation process are described.