The insertion of methylene into metal−alkyl bonds has been studied on an Ag(111) surface under ultrahigh-vacuum conditions. Methyl (CH3) and methylene (CH2) groups are generated on Ag(111) by thermal scission of the C−I bonds in adsorbed methyl iodide (CH3I) and methylene iodide (CH2I2) below 200 K. When studying the bimolecular interaction of coadsorbed CH3 and CH2 moieties, propane and butane are detected in the temperature-programmed reaction (TPR) experiments, suggesting that CH2 inserts into the Ag−CH3 bond to produce metal-bound ethyl (CH2CH3) groups, followed by facile self-/cross-coupling of existing alkyl groups on the surface. In a separate approach, CF3 is used instead of CH3 as the target for CH2 insertion. TPR studies of the coadsorbed CF3 and CD2 show that CD2 also inserts into the Ag−CF3 bond to form a new C−C bond; however, a different chain termination step (β-fluoride elimination) is observed to produce 1,1-difluoroethylene-d 2 (CF2CD2) in the gas phase. Two sequential methylene insertions are also implicated, as evidenced by the production of 1,1,1-trifluoropropyl iodide-2,3-d 4 (CF3CD2CD2I) as a result of the recombination of CF3CD2CD2 groups with surface iodine. The relative rates of methylene insertion, alkyl coupling, and β-elimination involved in the reaction mechanisms are systematically compared. The effect of coadsorbed iodine on the reaction pathways is assessed. The methylene insertion rate is found to be more facile on silver than on copper surfaces.