The relationship between diffusion and phase diagram is discussed in more detail for Ag-Au and Au-Ni alloys. For each alloy tracer diffusion and interdiffusion data have been compared with the corresponding phase diagram. Tracer diffusion is related to the solid-liquid phase transition. Ag and Au tracer diffusivities D*(x) in Ag-Au alloys and melting temperature T-m(x) of Ag-Au show opposite curvatures. The same holds for D* of Au and Ni in Au-Ni alloys and indicates a vacancy diffusion mechanism. Interdiffusion depends on the critical temperature of the solid-solid phase transition of the phase diagram. The curvature of interdiffusivity D(x) is determined by the sign of T-c(x) or the interaction energy epsilon of the regular alloy. The calculations agree well with the experimental data.