The aromatic polyester from diphenolic acid and isophthaloyl chloride was synthesized by interfacial polycondensation with tetrabutylammonium chloride as phase transfer catalyst. The structure was characterized by means of FTIR. The poly(DPA-IPC) solution was titrated by aqueous HCl solution. The titration curves were obtained. The trend of reduced viscosity of solution with concentration was studied. Besides, the stability of poly(DPA-IPC) in aqueous basic solution was discussed.