When O,O'-Bistrimethylsilyl catechol (BTSC) was polycondensed with adipoyl chloride in o-dichlorobenzene the 10-membered cyclic monomer was the main reaction product regardless of the concentration. Even the polycondensation in bulk. yielded the macrocyclic monomer as the main product. Polycondensations of free catechol yielded similar results. Polycondensations of catechol or BTSC with suberoyl chloride, sebacoyl chloride, and decane-1,10-dicarbonyl chloride in concentrated solutions or in bulk yielded cyclic oligoesters as the main reaction products whereas linear oligoesters or polyesters were a minority. Polycondensations of BTSC with the longer diacid dichlorides in refluxing o-dichlorobenzene under high dilution yielded the 12-14-membered cyclic monomers as the main products. The molecular weights and the cyclic structure of all reaction products were characterized by mass spectrometery, fast-atom bombardment (FAB) or MALDI-TOF mass spectrometry.