In tracer self-diffusion or impurity diffusion experiments heterovalent impurities initially present on the surface or in the deposited tracer may co-diffuse into a sample along with the tracer. In ionic crystals, even very tiny amounts of heterovalent impurity, comparable to the tracer concentration, may introduce extrinsic vacancies in the diffusion zone which markedly influence the measured diffusion profiles of the tracer. A cornputer fit model has been developed to provide an approximate solution for the case of a homovalent tracer and an unknown heterovalent impurity co-diffusing via vacancies in an ionic solid as well as for the case of a heterovalent tracer where the tracer itself iritroduces extrinsic vacancies. Solutions show a dependence of the profile shape on diffusion time and marked deviations from a Gaussian fit for a thin source, even for the deep penetration region, indicating that the limiting slope is not necessarily a valid measure of the tracer diffusivity. The effect is also shown to enter to first order in measurements of the isotope effect in ionic solids, so that small deviations may cause major errors in determination of the isotope effect. The program is used to fit experimental measurements of the pressure dependence of Na self-diffusion in NaCl and of the time dependence of the diffusivity, providing consistent values for the diffusivity of the tracer as well as for the amount and diffusivity of the unknown impurity