The kinetics and phase formation sequence of thin-film solid-state reactions, including silicide forming reactions, have frequently been considered to be controlled by interfacial kinetic reaction barriers. These are purely phenomenological quantities which describe the finite rate of the interfacial reaction in terms of limited particle fluxes crossing the respective interfaces. No atomic mechanisms that might be responsible for the action of such barriers have so far been indicated, with the exception of Schmalzried's formulation. The latter says that the interfacial barrier is due to the limited relaxation time needed by the particles to rearrange into the proper sublattice after having crossed the interface. We present correlated kinetic and structural observations during the 2Ni + Si → NiSi2 reaction on the Si(111) surface and discuss them with the help of a model involving the formation and lateral propagation of interfacial steps of different height. The model allows us to explain the kinetic observations by reaction barriers formed as a result of the crystallographic boundary conditions of the reaction.