This paper reports the preparation and characterization of sustained release sodium diclofenac microparticles by spray drying. Aqueous dispersions of ethyl cellulose (Surelease (R)) and Eudragit RS 30 D (R) were evaluated as controlled release polymers. The product properties (product Moisture, size distribution, particle morphology, flow properties, total drug load, in-vitro dissolution studies, and encapsulating efficiency) were determined as a function of inlet temperature of the spray drying, the feed flow rate and composition of the microencapsulating formulation. In general, lower values of the product moisture content were observed at higher drying temperatures. The spray-dried product was composed mainly by rounded-shape and multi-size particles. The mean particle diameters of the Eudragit based microparticles tended to be slight bigger than the Surelease based microparticles. The spray dried microparticles showed delayed drug dissolution rates, sustaining the drug release for several hours. These findings confirm the feasibility of the spray drying for preparation of microparticles with sustained release properties. The physical and chemical properties of the microparticles can be changed by varying the spray drying parameters as well as the microencapsulating formulation. (c) 2006 Elsevier B.V. All rights reserved.