We investigated catalytic activities of strong bronsted acids for dehydration polycondensations of dicarboxylic acids and diols, which were carried out at low temperature (<100 degrees C) under reduced pressure (0.3-3 mmHg). Strong Bronsted acids, bis(perfluoroalkanesulfonyl)imide and perfluoroalkanesulfonic acid, showed higher activity than p-toluenesulfonic acid or rare-earth catalysts at 60 C. In particular, bis(nonafluorobutanesulfonyl)imide (Nf(2)NH) showed the highest activity to synthesize not only aliphatic polyester (M-n > 19000) but also aromatic polyester (M-n > 7000). The used Nf(2)NH was sublimated from the reaction flask during polycondensation, and the sublimate, Nf(2)NH, was extra pure so that we can reuse the catalyst without loss of the activity in the dehydration polycondensations.