Abstract: A stable sustained release theophylline formulation is prepared by incorporating theophylline into a semi-solid matrix comprising polyglycolized glycerides (GELUCIRE® excipient) and a mixture of nucleation enhancers. Theophylline is admixed with molten GELUCIRE to make the sustained release formulation. The nucleation enhancer composition is then incorporated in the admixture to make the sustained release formulation resistant to changes in dissolution upon aging. Orally administrable compositions are prepared by filling gelatin capsules with the formulation. The polyglycolized glycerides (GELUCIRE) and the nucleation enhancer composition also can be used as an excipient system for preparing sustained release pharmaceutical compositions.