Formation and properties of binary compounds made by thin film interactions at temperatures well below the melting point of the various components were recently studied. Several aspects concerning such compounds are still, however, unclear; among which are (a) the mechanism responsible for the growth, (b) which phase is formed first, and (c) why not all the compounds predicted by the phase diagram were observed. We review the literature and suggest that the processes occurring at the interfaces control the phase formed. This conclusion is supported by new experimental data concerning the stability of the various phases as a function of the temperature and/or the sample composition.