A polymerization reaction may lead to either a thermoplastic or thermoset system depending upon the chemical structure of the monomers, and is usually accompanied by a marked increase in viscosity, which is directly related to the extent of reaction. As the molecular weight increases and branch chain structure develops, so a characteristic frequency dependence is observed in the rheology of the system. This paper describes the development of theoretical models which are capable of describing the behaviour of a system in which a condensation polymerization reaction is taking place. Polymerization of difunctional monomers leads to linear polymers, whereas higher functional monomers will ultimately form a three-dimensional network structure. Validation of the models used is achieved by comparing the prediction with experimental data for well deΓned stable polymer systems. Two sets of materials are studied : Γrstly, a series of narrow molecular weight linear polystyrenes and secondly, a series of polyisoprene star branched materials. The results of the simulations of the growth of a linear and a branched polymer system are presented, and the implications in relation to the form of the equations used are discussed.