A parametric study of a two-zone diffusion model has been performed to describe in-pile diffusion-controlled release of fission products from spherical coated fuel particles. Both the steady-state release and the times to reach steady state are considered. The effects of variations in diffusion coefficients of the fuel and coating, coating thickness, partition coefficient at the fuel-coating interface, contamination fraction, and decay constant have been considered. The results predict three regimes of release for different ranges of half-life and diffusion coefficients. Certain very long-lived isotopes will have high equilibrium release rates controlled by diffusion in the fuel core but probably will not come to equilibrium during the lifetime of a fuel particle. The release of many isotopes with intermediate half-lives is controlled by diffusion in the coating material. Equilibrium release rates in this range are large and probably will be achieved in practice for pyrolytic carbon coated fuel particles. The release of the inert gases is controlled by the level of fuel contamination in the coating material. The beneficial effects of using improved barrier coatings are discussed in terms of the diffusion model. Certain unusual aspects of the in-pile release of fission gases are explained in terms of the results of this model.