Analytical method to solve differential diffusion equations describing the growth of the phase wedge during the intermetallic-compound formation with a narrow concentration range of homogeneity in bicrystals is proposed. A model describing the diffusion phase growth from point source inside the polycrystal grains is regarded. Analytical method to solve differential diffusion equations for such a model is suggested. Parabolic, cubic, and fourth power diffusion regimes for different scales from nanometers to micrometers and millimeters are analysed.