Adsorption and metalation of porphyrins on metal single-crystal surfaces under ultrahigh vacuum has been studied extensively, but less is known about the adsorption behavior of porphyrins on oxide surfaces. On metal surfaces porphyrin molecules will react with coadsorbed metal atoms or the surface itself, producing the metalated molecules and coadsorbed hydrogen atoms, which subsequently combine and desorb as hydrogen molecules. Examples will be given of 5,10,15,20-tetraphenylporphyrin (2HTPP) on Cu(111). A similar reaction is also possible for oxide surfaces. As 2HTPP is adsorbed on MgO an ion-exchange reaction happens already at room temperature and the two aminic protons of the porphyrin molecule are exchanged with a Mg²⁺ ion from the oxide surface, producing MgTPP and, locally, Mg(OH)₂. Examples will be given of adsorption on both single-crystal MgO(100) substrates and nanostructured powders. Examples will also be given of metalation of 2HTPP adsorbed on a Au(111) surface immersed in an aqueous zinc acetate solution, metal-exchange reactions with Cu²⁺ and unsuccessful attempts to demetalate the adsorbed metalloporphyrins in acidic solutions.