Polyester resins are prepared by reacting a lower alkyl ester of an aromatic dicarboxylic acid with an aliphatic diol to form an aromatic polyester, and which includes (i) subjecting to transesterification reaction conditions in the presence of a titanium compound catalyst a transesterification reaction system having the lower alkyl ester of the aromatic dicarboxylic acid, the aliphatic and between 0.02 to 5.0 mole percent, based on the amount of the lower alkyl ester of the aromatic dicarboxylic acid, of an aromatic sulfonic acid compound represented by the following general formula (I):OH--R--O--Ar--SO.sub.3 M (I)wherein Ar is a p-substituted benzene group or a 2,6-substituted naphthalene group, R is a divalent group selected from among --CH.sub.2 CH(CH.sub.3)--, --CH(CH.sub.3)CH.sub.2 --, --CH.sub.2 CH(CH.sub.3)-- and --CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 --, and M is an alkali metal selected from the group consisting of lithium, sodium and potassium. The resulting transesterification reaction product is thereafter subjected to polycondensation to obtain the polyester resin. The polyester resin may be blended with an aromatic polyester base resin to yield a moldable polyester resin composition having exceptional crystallization rates.