Matrix Diffusion Controlled Systems : Here, the drug is dispersed in an insoluble matrix of rigid non swell able hydrophobic materials or swell able hydrophilic substances. Materials used for rigid matrix are insoluble plastics such as PVC and fatty materials like stearic acid, beeswax, etc. With plastic materials, the drug is generally kneaded with the solution of PVC in an organic solvent and nucleated. Waxy matrix is prepared by dispersing the drug in molten fat followed by congealing. The granules are then compressed into tablets.