The dynamic swelling of gelatin/poly(acrylic, acid) microspheres loaded with oxprenolol has been evaluated. The movement of two distinct and characteristic swelling boundaries was measured directly using an optical microscope. Swelling rate constants associated with the inner moving front and the outer swelling boundary were determined. A polynomial equation incorporating both Fickian diffusion and case II relaxational models was used to fit the kinetics of liquid uptake. The influence of the concentration of the cross-linking agent, the poly(acrylic acid) load, the pH of the swelling medium and the particle size of the microspheres, on the swelling kinetics, was evaluated and discussed.