The carbonaceous layer formed on the surface of a MoO3 model catalyst during propylene metathesis is examined using Auger electron spectroscopy and ex situ laser Raman spectroscopy. Auger spectroscopy confirms the formation of a carbonaceous layer similar to that found after catalysis performed by Mo metal and MoO2. Raman spectroscopy indicates that the carbonaceous layer is primarily composed of surface-bound hydrocarbon fragments and small gaphitic particles.