A mathematical model of simultaneous dissolution and diffusion controlled drug release from swellable, non-erodible viscoelastic polymer matrices is developed. The model assumes that the drug is loaded into the dry matrix as a solid phase. Two unsteady-state mass balances are required for both the drug and the incoming external penetrant. The Camera-Roda and Sarti equation is considered for the determination of the penetrant uptake, while Fick＇s equation accounting for the density gradient developing in the matrix is considered for the drug diffusion. Assuming that upon dissolution a drug phase transition may occur which affects the drug solubility without altering the diffusion coefficient. This analysis revealed that the features of the drug release kinetics are sensibly affected by the viscoelastic properties of the swollen matrices and by the drug dissolution and phase transition rates.