Transition and rare-earth metals have been found to interact with single-crystal Si in a way that allows a division into three distinct classes: near noble, refractory, and rare earth. Recently, attention has turned to the reaction of their binary alloys with Si. In this paper we will try to demonstrate that by regarding the alloy-Si reaction as a phase transformation of alloys under the influence of a reactive substrate, we can undertake a systematic approach for the study of this kind of phase transformations involving ternary elements. In essence we show that the kinetic path taken by the alloy-Si interaction can be understood and anticipated from the reaction characteristics of the proper metal/Si bilayers and the reaction in the alloy itself. Results will be shown for Er-Pt, and Gd-Ti alloys on Si which confirm this systematic approach, which is also supported by previously published data.