A differential equation describing interstitial‐substitutional diffusion is derived. Boundary conditions are selected pertinent to diffusion into a finite medium. Laplace's tranformation is used to solve the differential equation. The result is plotted as a relation between dimensionless quantities. The quantities D1 and C01 can be calculated from the intercept and the initial slope, respectively. The theory is applied to the diffusion of gold into silicon wafers of varying thickness.