Cobalt was electrodeposited onto a reconstructed Au(100) electrode in 0.25 M Na(2)SO(4) + 0.1 mM CoSO(4) under potentiostatic conditions. The Au(100) electrode loaded with the Co deposit was transferred via an argon environment to ultrahigh vacuum and subsequently examined with reflection high-energy electron diffraction (RHEED). The resultant RHEED results were analyzed in detail to unveil the structures of the as-prepared Co deposit, which assumed nanometer-sized clusters on the Au(100) substrate. Aided by coulometric results, profile analysis of the RHEED results indicated that the Co clusters on average were 3 nm in width and 15 atomic layers in height. The packing habits of the Co clusters, determined by the obtained RHEED results, were the face-centered cubic (fcc) phase, rather than the hexagonal closed-packed (hcp) phase. Cobalt clusters grew with their fcc ((1) over bar 10) plane aligned parallel to the Au(100) substrate and with their  axis aligned in the  direction of the Au(100) substrate. The preferential growth of the fcc Co phase is driven by the optimal lattice match between the ((1) over bar 10) plane of Co clusters and the Au(100) substrate network.