A new hypothesis is presented saying that an ideal thermodynamically controlled polycondensation is characterized by the following three aspects: first, the ring-chain equilibria automatically include ring-ring equilibria which become decisive for the thermodynamical properties of the system at high conversions. Second, at 100% conversion, all reaction products are cycles. Third, the chain growth is limited by the thermodynamical properties of the ring-ring equilibrium. The following polycondensations were discussed: base-catalyzed transesterification of ethyl cholate and combinations of Bu2Sn-(OMe)(2) with alpha,omega-poly(ethylene glycol)s, with poly(tetrahydrofuran)diols or with two oligosiloxane diols. Moreover, equilibrations of oligo- and polysiloxanes and polycondensations of Bu2Sn bisacetate with various aliphatic dicarboxylic acids are discussed. Furthermore, polycondensations of pentaerythritol and its oligoether derivatives with Bu2Sn(OMe)(2) yielding spirocycles are discussed. It is demonstrated by a variety of chemical or analytical methods that Sn-containing polymers were never formed and that cycles were the only reaction products. These results clearly support the above formulated modifications of the Jacobson-Stockmayer theory and of the Carothers-Flory theory.