The formation and hydrogen removal of the thick carbonaceous layer accumulated during metathesis of propylene at 880 R on molybdenum and molybdenum oxide model catalysts is examined, It is determined that a hydrocarbon layer several monolayers thick forms rapidly during reaction, and that a portion of tills layer may be easily removed by healing the used catalyst in hydrogen. An activation energy of 6.4 +/- 0.4 kcal/mol is observed fur this process. A graphitic layer also forms slowly on the catalyst surface during reaction, and tills layer is difficult to remove by reaction with hydrogen. It is demonstrated that addition of hydrogen to the metathesis reaction mixture reduces the thickness af the carbonaceous layer on the catalyst surface during reaction, thereby enhancing the observed metathesis rate.