We present a combined helium atom scattering (HAS) and medium energy ion scattering (MEIS) study to provide a detailed description of sub-monolayer growth of copper on vicinal platinum. Initial growth is epitaxial but induces a small relaxation in the substrate. A strong temperature dependence of growth is observed and MEIS indicates the formation of a thin (two-layer) alloy between 450 K and 500 K . In-situ HAS measurements allow for the direct observation of alloying and reveal that it occurs in two unusually sharp phase transitions. Both HAS and MEIS indicate the onset of bulk alloying and dissolution of copper at surface temperatures above 600 K.