Enzyme-catalyzed synthesis of aliphatic, unsaturated, and semiaromatic polyesters from diesters and diols was carried out in toluene at different temperatures in the presence of supported lipases from Candida antarctica. Whatever the monomer structure, rings were formed concurrently with linear chains except when dimethyl fumarate was used. The yield of cyclics depends on several parameters such as the monomer structure and the initial concentration of the reactive functions and strongly influences the average molar masses of unfractionated samples. Qualitatively, this is a general feature of the ring-chain equilibrium. Once the existence of this equilibrium was shown, the molar cyclization equilibrium constants (K) for aliphatic, unsaturated, and semiaromatic polyesters were determined, and it would appear that the molar distribution of the ringed species obeys the Jacobson-Stockmayer equation (ln K = ln B - 5/2 ln n, where B and n represent a constant and the number of monomer units of each type in the ring, respectively) except when n is equal to 1. The so-called cyclic dimer exception is discussed in relation to the catalyst's enzymic nature.