The continuum Fokker-Planck formalism is applied to describe the compositional changes in composite solids during simultaneous ion bombardment and atomic flux deposition. The general defining equations are derived. As a demonstration of the limitations and the extent of applicability of this approach the case of ion irradiation of composite solids has been studied comprehensively in the limit of ballistically equivalent species and in the absence of cascade mixing and diffusion. In the second part of this communication some general relations between the steady state partial sputtering yields of the different constituents for some specific systems are also derived. In particular, the net erosion and the net deposition regimes are studied as well as the transition between them. It is shown that the latter could be discontinuous with increasing deposition fluxes for particular systems.