The growth of Ag on Pt(111) and the formation of a nanophasic alloy, has been thoroughly studied. On the Pt(100) surface, the presence of the energy-minimizing, hex-reconstruction gives rise to phenomena different to that on the (111) face. We present STM, AES, and TPD studies of the Ag/Pt(100) interface for submonolayer Ag coverages in a temperature regime between 300 and 900 K. We observe strongly anisotropic growth of Ag islands governed by the anisotropy of the reconstructed surface at 300 K. Adsorption of Ag lifts the reconstruction, resulting in a kinetically trapped Ag80Pt20 alloy formed by using the excess Pt from the reconstruction ejected into the growing Ag-islands. Annealing causes diffusion of Ag from the Ag80Pt20 adislands to the pure Pt(100) reconstructed regions, lifting its reconstruction and forming a nanophasic Ag-Pt alloy. For surfaces with less than 0.3 ML Ag, domains of Pt(100)-hex reconstruction and a Ag30Pt70 alloy coexist in thermodynamic equilibrium. This phase segregation is a result of energy minimization, and can be explained by the presence of two minima in the Ag-Pt surface phase diagram associated with the pure Pt reconstruction and a strain stabilized alloy phase. Deposition of more than 0.3 ML increases the Ag content in the alloyphase until no Pt remains in the monolayer and an abrupt Pt/Ag interface forms.