Recently, coated pellets have gained attention of the pharmaceutical industry as they represent a relatively easy way leading to controlled drug release. Pellets of appropriate properties containing approx. 40% of diclofenac sodium were prepared by roto-agglomeration. For the coating, two different aqueous dispersions (Surelease and Eudragit RS 30 D) were selected. Generally, the drug release rate slowed down as the coating load increased from 10 to 22%. However, big differences between diclofenac sodium release from pellets coated with Surelease and Eudragit RS 30 D at equivalent coating loads were observed. Although Eudragit RS 30 D provided membranes successfully controlling drug release over an extended period of 24 hours, the coating process with Surelease led to a film of a very poor quality. Faster release from ethyl cellulose coated pellets could be explained in terms of a higher solubility of diclofenac sodium in alkaline aqueous dispersion of ethyl cellulose and its migration into the coat during the coating process and/or the unsatisfactory curing of the ethyl cellulose film. Therefore possible interactions between the coating and the drug should be always considered as suggested in this study.