In the formation of a coated controlled release preparation with functional coat layers, hydroxypropyl-methylcellulose was used to form a diffusion layer which swelled immediately upon wetting. Eudragit RS30D was used to form the outer retention layer. The rupture of pellet coat occurred when the Eudragit RS30D was unable to withstand the expansion in volume due to the influx of water and swelling of the hydroxypropylmethylcellulose diffusion layer. The sucrose core was able to contribute an osmotic effect. The hydrostatic pressure built up within the pellet can cause the pellet coat to rupture. Sodium chloride deposited in the diffusion coat was able to delay the bursting of the pellet coat. This was due to the competition for the imbibed water between sodium chloride and hydroxypropylmethylcellulose. The rupture of the pellet coat did not result in a total failure of the controlled drug delivery mechanism. Similar drug release rates were obtained irrespective whether there was a puncture in the pellet coat or not. Pressure built-up in the region away from the puncture pushed the core material towards the point of puncture and sealed the puncture point. In addition, the swelling of polymer around the point of rupture ensured continuity in the drug diffusion barrier.