An investigation was carried out to discover the effects of high conversion on the kinetic rate coefficients for the polymerisation of styrene. In such regimes, the reaction medium is of such high viscosity that these coefficients (kp and kt) exhibit profound changes. The changes were investigated with respect to the viscosity of and volume fraction of polymer present in the reaction medium. To circumvent the need to polymerise the monomer from the zero conversion, pre-formed polystyrene of known molecular weight was dissolved in purified styrene monomer. The resulting viscous solutions were then polymerised with AIBN as initiator. Effects on kp, kt and a reduction in f, initiator efficiency, were noted. Volume-fraction based models of termination kinetics were shown to be less applicable than one based on viscosity.