The cross coupling of phenyl groups (C6H5) With alkyl iodides in adsorbed monolayers on single-crystal Cu(110) and Cu(100) surfaces under ultrahigh-vacuum conditions has been studied by a combination of temperature-programmed reaction techniques and isotope labeling. In these experiments, phenyl groups were generated on the surface by the dissociative adsorption of iodobenzene. Detailed studies in the case of the CH3I + phenyl reaction to form toluene show that there are two mechanisms for the process : a low-temperature pathway (<160 K; activation energy <10 kcal/mol), in which phenyl groups react directly with CH3I, and a much higher temperature pathway (similar to 400 K, activation energy similar to 27 kcal/mol), in which adsorbed methyl groups produced by C-I bond scission couple with coadsorbed phenyl groups. The mechanism of the low-temperature reaction has yet to be fully resolved, but experimental evidence for a radical mechanism is presented. The relevant observations in this regard include the following : (1) when methyl radicals are impinged onto a phenyl precovered Cu(100) surface at 100 K, toluene is produced by an Eley-Rideal mechanism; (2) independent studies have shown that methyl radicals are produced during CH3I dissociation on Cu(111); (3) the upper limit of 160 K for the phenyl + CH3I reaction temperature on Cu(110) is quite close to the temperature for C-I bond scission in CH3I on this surface; and (4) neopentyl iodide, which is much more sterically hindered than CH3I, also shows a direct reaction with coadsorbed phenyl groups, suggesting that the process is C-I dissociation followed by coupling as opposed to phenyl attack at the C-I bond.