To determine the optimal structure and size of a cluster suitable for modeling chemical processes on the (1 It) surface Of Pt3Ni alloys and to understand how alloying changes the electronic structure, we used the B3LYP flavor of density functional theory (DFT) to study systematically the composition and the characteristics of the Pt/Ni(I 11) alloy surface for clusters up to 44 atoms. We find that the bulk structure (Pt3Ni crystal) has the atoms cubic closest packed as in bulk Pt or bulk Ni, but ordered such that each Ni has 12 bonds to Pt atoms and we find that this same pattern is preferred on the (I 11) surface. As a result, the ordered (I 11) surface has each Ni surrounded by six Pt atoms. After determining the optimal cluster, we examined the chemisorption of atomic hydrogen and atomic oxygen at all on-top (mu(1)), bridge (mu(2)), and 3-fold (mu(3)) sites on the (I 11) alloy surface. We find that oxygen binds most strongly at a fcc site (two Pt and one Ni atom) with an adsorption energy of 3.50 eV, which is 0.22 eV stronger than binding to the pure Pt(I 11) surface. The barrier for the 0 to migrate about fcc sites shared by the same Ni is 0.23 eV, but to hop over to a site bordering a different Ni is 0.53 eV. In contrast, the barrier on Pt(1 11) is 0.56 eV. Thus oxygen is much more localized on the Pt/Ni alloy surface than on pure Pt. A similar behavior was observed for the chemisorption of hydrogen. Here we find that the best site is on-top of Pt with D-e = 2.67 eV, which is comparable to pure Pt(I 11). In contrast to Pt(I 11) where the barrier for migration through the bridge and fcc sites is only 0.06 eV, making H very mobile, we find that on Pt/Ni(l 11) the adsorption energies range from 1.78 to 2.67 eV. Here the migration of the H from one Pt site to the next goes through the bridging and fcc sites sharing a Ni with a barrier of 0.12 eV, twice that for Pt(I 11).