The structures of the two-dimensional layers formed in the underpotential deposition of lead onto carefully prepared single crystals of copper are shown to depend upon the substrate orientation for (111), (100) and (110) surfaces. Nucleative growth processes are observed for first order and for higher order phase transitions. Comparison is made with the behaviour observed on silver substrates and the differences are ascribed to the change in atomic radius and in eletronegativity. Formation of the underpotential layer is shown to be a necessary precursor to overpotential deposition, the mechanism of which is markedly dependent on substrate orientation.