A numerical model for interdiffusion in HgTe/CdTe systems based on fundamental point defect mechanisms has been developed. The model includes continuity equations for the flux of Hg and Cd on substitutional sites, cation vacancies, and Hg and Cd interstitials. Interdiffusion is modeled by simulating the coupled diffusion and interaction of these species. The Hg vacancy concentration used in the model was fit to measured hole data as a function of annealing temperature, Hg pressure, and composition. The Cd and Hg interstitial diffusion coefficients and concentrations were determined as a function of temperature and composition from low temperature Hg and Cd self-diffusion data. With this model, interdiffusion is simulated over a range of initial and annealing conditions. At high temperatures and/or Te saturated conditions, interdiffusion is dominated by diffusion via a vacancy mechanism. Interdiffusion is controlled by the flux of Cd interstitials at lower temperatures and/or higher Hg pressures.