Copolycondensations of IPA, TPA, bisphenol A (BPA), and several cimonomers were carried out to improve thermal properties, such as, the glass transition temperature (T-g) of the IPA/TPA (50/50)-BPA polyester. Among the comonomers examined, 4,4'-Dihydroxydiphenylsulfone (BPS) and 4,4'-Dicarboxydiphenylsulfone (DCDPS) having a strongly dipolar sulfonyl group in the chain were significantly effective. The favorable effect upon the T(g)s was studied by varying the amounts of BPS and DCDPS incorporated into the copolymers. In the copolycondensation with BPS, two-stage copolycondensation of BPA first and then BPS, the reverse order of reaction, and their spontaneous addition were examined to investigate the effect of distribution of the BPS unit segments in the copolymer upon the T(g)s of the resulted copolymers. The distribution was briefly studied from distribution of the IPA/TPA-BPA oligomers in the initial reaction using GPC.