Abstract: AIM　To prepare 5-aminosalicylic acid (5-ASA) pellets for colon-specific drug delivery with water-based coating processes. METHODS　5-ASA time-controlled release pellets for colon delivery were prepared in a fluid-bed coater, with low-viscosity HPMC as the inner swelling layer and ethylcellulose aqueous dispersion-Aquacoat as the outer controlled membrane, and triethyl citrate added to the dispersion as the plasticizer. The in vitro release of pellets was investigated in different pH media and changing pH media for simulating gastrointestinal delivery. RESULTS　Addition of the swelling layer was necessary for making the time-controlled colon-specific pellets. Drug release was triggered by rupturing the outer membrane. The lag time of drug release was shortened to a certain extent with increasing amount of swelling layer. The lag time was prolonged significantly with the increase of the thickness of the outer membrane and of the amount of the plasticizer. The data of in vitro release showed that the higher the medium pH, the faster the release of 5-ASA from pellets. The drug release was initiated after a lag time of 5 h and was completed within the following 10 h for the pellets with a 4.5% w/w HPMC coating level, a 15% w/w ethylcellulose coating level and 25% w/w plasticizer based on Aquacoat solid. CONCLUSION　The time-controlled pellets for 5-ASA colon-specific delivery could be prepared using ethylcellulose aqueous dispersion by changing the swelling layer and controlled membrane coating levels.