A phenomenological methodology has been used to characterize the intrinsic diffusion of tin in GaAs based on the coupled motion of substitutional and interstitial Sn atoms. Both the rapid diffusivity of interstitial Sn atoms and their transformation to substitutionals by occupying gallium vacancies are processes that lead to an anomalous “double-profile” diffusion curve observed in highly doped samples. The model was developed for samples annealed over a range of 973–1123K. The model developed while developed and perhaps only applicable over this temperature range may be extendable to a larger temperature range provided that the underlying mechanism is unchanged. The methodology consists of using key aspects of the diffusion profiles along with the mass conservation criterion to predetermine bounds which prescribe the values of the parameters used to model the experimental data. This fitting procedure allowed for a more rapid assessment of the model parameters by providing physical bounds on their values as derived from the experimental profiles. The proposed diffusion mechanism combines the simultaneous diffusion of both interstitial and substitutional Sn atoms as well as key defect reactions characterizing their mutual equilibrium with the background vacancy concentration.