In this paper a thermodynamic model for a polymerized unary liquid is presented. The model assumes there is a distribution of species as a result of a polymerization equilibrium which involves chain formation and scission. According to the model, the distribution of species depends on the Gibbs energy of bond formation and on the temperature of the liquid. Such a polymerization equilibrium is widely believed to occur in the Se-Te melts. The paper discusses the heat capacity, which is compared with the experimental heat capacity of the Se-Te melts. A very close behavior is found and an explanation from the point of view of a polymerization equilibrium is suggested. (C) 1997 American Institute of Physics.