The aim of this study was to investigate liquid penetration into both cylindrical and convex hydrophobic matrix tablets and to relate the changes in tablet structure to drug release. Starch acetate with degree of substitution of 2.7 was used as a hydrophobic matrix former and anhydrous caffeine as a freely soluble model drug. Phenolred was used as a colouring agent to enhance the visual detection of the liquid boundary movements, which were examined in axial and radial directions for both types of tablets. The tablets started to expand during the dissolution, resulting in cracking as the liquid boundary penetrated into tablet. The cracking influences drug release by shortening the diffusion path and decreasing the tortuosity. The liquid boundaries proceed differently in cylindrical and convex tablets, this being attributable to differences in pore structure and density distribution. Cylindrical tablets are quite homogeneous in terms of density, but convex tablets have more porous areas at the domes of the tablet.