A reactive diffusion model of the high-temperature sequential deposition process is proposed, and applied to the formation of the icosahedral quasicrystalline Al4Mn phase, using recent results about the quasicrystal thin film formation (P.B. Barna, Phys. Scr., T49 (1993) 349). With this model, it is shown that a general analytical description of the physical processes characterising the formation of alloy phases by reactive diffusion is possible, and that concrete evaluations can be derived. A correlation between three quantities, diffusion coefficient, solubility factor and critical thickness, is given. This model and cross-sectional transmission electron microscopy observations make it possible to determine the diffusion coefficient of the mobile Al atoms in the Al4Mn structure by a non-conventional method.