It is shown that model oxides grown on metallic substrates catalyze propylene metathesis to form ethylene and butene with an activity that mimics that of supported catalysts for reaction below similar to 650 K. Another reaction regime is found for olefin metathesis above similar to 650 K, where the reaction proceeds with a much higher activation energy of similar to 60 kcal/mol. Unfortunately, alkenes do not react to any detecable extent on the model oxide surfaces in ultrahigh vacuum. However, the high-temperature(> 650 K) metathesis rate is found to be affected by the presence of oxygen overlayers, which also modify the chemistry of alkenes on Mo(100) in ultrahigh vacuum. It is found that methane is formed by reaction of alkenes on O/Mo(100). The only other products detected are ascribed to either hydrogenation or total thermal decomposition into carbon and hydrogen. It is shown, using iodine-containing molecules to graft hydrocarbon fragments onto the surface, that alkenes can dissociate, forming carbenes which react to yield methane. This chemistry is in accord with that found catalytically at high temperatures, where the product distribution from the reaction of ethylene is well described by a Schulz-Florey distribution and the product distribution from propylene is well described by copolymerization of carbenes and methyl carbenes.