Polycondensations of aliphatic dicarboxylic acids (ADAs) with 1,4-butane or 1,6-hexane diol were conducted in bulk at 80 or 100 degrees C. Temperature and monomer/catalyst ratio were varied. With bismuth triflate, Bi(OTf)(3), as catalyst, number average molecular weights (M-n'S, uncorrected SEC data) up to 30 000 Da were obtained. Analogous polycondensations were performed with the triflates of sodium, magnesium, aluminum, zinc, tin(II), scandium, and hafnium, but the highest M-n values were achieved with Bi(OTf)(3). Polycondensations and model experiments proved that most triflates catalyzed the formation of tetrahydrofuran from 1,4-butane diol above 100 degrees C. Therefore, polycondensations of 1,4-butane diol required temperatures <= 100 degrees C. Polycondensations of 1,4-butene diol or 1,4-butyne diol were unsuccessful, because of side reactions. All polycondensations involved rapid equilibrations, and the content of cyclic oligo- and polyesters increased with the conversion.