Pd-Ge reactive-diffusion in two different samples Pd/Ge(001) and Pd/PdGe/Ge(001) was studied experimentally and theoretically, in order to clearly identify the driving force controlling the first-phase selection in the sequential phase formation regime. The experimental and theoretical results both show that only the atomic transport kinetic in the phases controls the first -phase selection: the phase exhibiting the fastest atomic self-diffusion forms first, even if its formation energy is not the lowest. (C) 2016 Elsevier Ltd. All rights reserved.