Adsorption of porphyrins on metal surfaces has been studied extensively in ultrahigh vacuum, but, despite the technological importance of oxides, much less work exists on the adsorption of porphyrins on oxide surfaces. Using X-ray photoelectron spectroscopy (XPS) the adsorption and reactions of 5,10,15,20-tetraphenylporphyrin on MgO(100) and TiO₂(110) surfaces in ultrahigh vacuum have been studied and will be compared with reactions on metal surfaces both in ultrahigh vacuum and immersed in aqueous solution. Focus will be on metalation of free-base molecules and metal center exchange of metalloporphyrins where the porphyrin molecules extract metal ions from the oxide surfaces. The behavior on oxide surfaces will be shown to have a lot in common with reactions in the liquid phase, such as ion exchange and protonation.