Sulfamethoxazole was microencapsulated by a gelatin-acacia coacervation method. The zeta potential of the microcapsules prepared at various coacervation pH and the amounts of formaldehyde used as a hardening agent were measured in the buffer solutions with pH, 2.5 to 9.5 and ionic strength, 0.1. The formalization affected the electrophoretic properties of the microcapsules, which depended upon the amounts of formaldehyde used. A pretty amount of formaldehyde (e.g. 30ml) raised the zeta potential as compared with that of the unformalized one. While a plenty amount of formaldehyde used (e. g. 50ml) tended to reduce this action. This characteristic phenomena were caused to the different two actions of fromaldehyde, i. e. denaturalization of gelatin and liberation of acacia from the microcapsule wall.The zeta potential of the original sulfamethoxazole and the coacervate droplets were also measured. By following the changes in the zeta potential in due course of the preparation of microcapsules, the preparation process of microcapsules was elucidated electrophoretically. The zeta potential of spraydried microcapsules indicated that the gelatin in the microcapsule wall was denaturalized by spray drying.