The reaction of ethylene catalyzed by metallic molybdenum at 875 K yields higher hydrocarbons (>C-2) with a distribution that is well described by a Schulz-Flory plot suggesting that their formation arises from the polymerization of surface C-1 species. This observation is in accord with results from surface science experiments that show facile C=C cleavage in alkenes. A similar molybdenum-catalyzed reaction of propylene yields metathesis and hydrogenolysis products. In view of the product distribution found with ethylene, these products are proposed to arise from the recombination of surface species formed by propylene dissociation. This result is confirmed by modifying the Schulz-Flory function assuming equal surface coverages of carbenes and methyl carbenes which adequately predicts the measured product distribution.