The topic of this paper is the experimental and theoretical study of drug-release from a system of polydisperse microencapsulated particles that, for the sake of simplicity, are assumed to be spherical. The theoretical analysis performed yields of a mathematical model to describe the physical phenomena involved in drug-release from such a system. In particular, the model is based on the hypothesis of a progressive dissolution of the internal solid drug core (due to solvent penetration through the coating) that gives a liquid solution in the region between the coating and the dissolving solid core. The existence of a concentration gradient between the inner solution and the outer release environment determines drug diffusion through the coating. The coacervation technique was adopted to microencapsulate the solid drug cores (theophylline, a bronchodilator for the treatment of chronic asthma and chronic obstructive lung disease) by an insoluble polymeric layer of ethylcellulose. The amount of drug released from these microencapsulated particles to the external receiver phase is monitored by means of a UV spectrophotometer. As the proposed model fits the experimental results well, it was concluded that it can be a good tool to design and to study this kind of drug-release system.