In this paper, we report on the formation of epitaxial copper silicide (Cu3Si) islands and their subsequent shape transitions on Si(111) and Si(100) substrates. For island growth on Si(111), equilateral triangles have been found to grow up to a critical size, beyond which a shape transition to trapezoid occurs. Similarly, on a Si(100) surface, square islands are observed which transition to rectangular islands and long nanowires. Initial growth on all substrates appears to be contingent on void formation that is tied directly to sustained high temperature processing of the samples. Overall, the island area and density are consistent with diffusion-driven growth and energetic barriers extracted from temperature-dependent data are in line with those seen in other studies.