The olefin metathesis activity of a series of model catalysts consisting of metallic molybdenum, MoO2, or MoO3 is investigated using a combined high-pressure reactor/ultrahigh vacuum apparatus. The model catalysts are analyzed using X-ray and ultraviolet photoelectron, Raman, and Auger spectroscopies and it is shown that MoO2 is the most active above similar to 650 K. Reaction in this temperature regime proceeds with a high activation energy (similar to 60 kcal/mol). Another, lower-activation-energy (similar to 6 kcal/mol) regime is encountered for reaction below similar to 650 K and it is suggested that both reactions proceed simultaneously, Ln addition, the absolute rate and selectivity for the reaction on MoO2 below 650 K are similar to those found for high loadings of supported molybdenum oxide, suggesting that an oxide layer provides a good model catalyst for this reaction. Pt is also found that reaction proceeds in the presence of a thick carbonaceous layer where the thickness of this layer decreases with increasing oxidation state, A Raman analysis of the surface of the catalyst following reaction indicates that this layer consists predominantly of hydrocarbon species, although a small amount of graphitic carbon is detected.