Some possible effects of elastic stress, engendered by epitaxial strain and compositional inhomogeneity, on the kinetics of intermediate phase formation in a planar thin‐film binary diffusion couple are identified. A simple quasistatic approximation to the diffusion equation is used to estimate the thickening rate of the intermediate phases when elastic effects are incorporated into both the diffusion equation and the interfacial boundary conditions for diffusion using the thermodynamics of stressed solids. The various growth regimes of the phases are visualized using a simple graphical representation of the phase growth velocities (phase plot). Growth kinetics are shown to depend on both the strain engendered by and the crystallographic orientation of the substrate. It is also proposed that elastic stresses could influence the sequence of phase formation in thin‐film systems including such observations as the absence of those phases appearing in the equilibrium phase diagram and the initial disappearance and subsequent reappearance of equilibrium phases.