The formation of new solid phases by diffusion between two interacting elements is analyzed on the basis of observations made (by others) on the well characterized aluminum-nickel system. Variations of the diffusion coefficients with composition have dramatic effects on the growth of AlNi and Al3Ni2. These phases are split into relatively sharply defined regions of different compositions, but not, as one may think, into separate phases. The effects of internal stresses, due to changes in lattice parameters with composition, on the actual composition profile of a growing phase are analyzed in the light of considerations relating to parallel effects of stresses on diffusion in simple solid solutions. The similar results reported for aluminum-nickel reactions carried out at 1000°C and 220°C provide the motivation for a review of what is known about lattice and grain boundary diffusion in intermetallic compounds. The conclusions are presented in the form of rules and observations believed to be generally valid for all diffusive reactions in the solid state. © 1992.