A new model for the erosion kinetics of semicrystalline surface-erodible homopolymers and copolymers is presented. The model is derived for a class of surface-erodible polyanhydride copolymers, with the goal of describing erosion in terms of fundamental, elementary processes. This model is based on an accurate description of copolymer microstructure and can thereby account for the heterogeneous erosion due to microphase separation and crystallinity. In addition to accurately predicting the overall erosion profile and the release of individual monomer species, several key phenomena that occur during erosion are described. These include precipitation of slightly soluble degradation products inside the pores of the erosion zone and pH changes during erosion due to dissolution of acidic monomers and the consequent changes in monomer solubility. This model also motivates future experiments to investigate predicted phenomena such as the effects due to local changes in pH and degradation rate constants for crystalline and amorphous moieties. The rational design of biomedical devices such as vehicles for drug delivery and scaffolds for tissue engineering will be aided by the application of this model and future extensions of it.