Using allyl halides (C3H5X, X = Cl, Br) to generate adsorbed C3H5 fragments and halogen atoms, we have studied the isomerization and reactivity of sigma -bonded (eta (1)) and pi -bonded (eta (3)) C3H5 on two metal substrates, Cu(100) and Ag(111), using vibrational and thermal desorption spectroscopies. On Cu(100), two forms of eta (3)- C3H5, denoted endo-eta (3)-C3H5 <----> exo-eta (3)-C3H5, are identified that undergo a reversible temperature-dependent isomerization. When dissociative adsorption of C3H5X saturates on Cu(100), only eso-eta (3)-C3H5 is found between 77 and 300 K. On Ag(111), eta (1)-C3H5 and eta3-C3H5 (both exo and endo) form at 110 K, depending on the dose. The eta (1)-C3H5 and exo-eta (3)-C3H5 irreversibly isomerize to endo-eta (3)-C3H5 upon annealing to 175 K. When either metal substrate, held at a selected temperature between 77 and 150 K, is dosed with C3H5Br, there is a reaction with eta (3)-C3H5 to form 1,5-hexadiene, C6H10 This reaction does not occur upon dosing with C3H5Cl. When eta (3)-C3H5 is heated above 200 K, different results obtain for the two metals: on Cu(100), in agreement with previous work, C3H5 radical desorption at 450 K dominates, while dimerization and desorption as C6H10 at similar to 250 K occurs on Ag(111).