Based on the theory of irreversible thermodynamics explicit expressions are derived for the entropy production during reversible polymerization of bifunctional linear polymers whose initial molecular weight distribution can be chosen arbitrarily. The time-dependent course of the entropy production is explicitly calculated for two cases where the reaction starts (a) from monomer and (b) from monodisperse polymer molecules. In both cases we treat the system to be ideal and the time dependant change of the number of molecules is described by a kinetic approach using two kinetic constants for the forward and backward reactions, respectively. During reversible polymerization the entropy production sigma(red) is a monotonously decreasing function approaching zero when the system reaches the equilibrium molecular weight distribution with sigma(red) being positive in accordance with the second law of thermodynamics. In case of starting reaction from monodisperse polymer molecules under constraint that the number average chain length remains constant during reaction we calculate the entropy of mixing and discuss it with results obtained from statistical considerations. (C) 2003 American Institute of Physics.