A viscometer-reactor assembly is used to generate data on the viscosity, eta(t), of an example polymerizing system exhibiting the Trommsdorff effect, namely, the bulk free radical polymerization of methyl methacrylate (MMA), at different temperature conditions [near-isothermal and non-isothennal (near-step increase and near-step decrease in temperature)] and at two different initiator, 2,2'-azoisobutyronitrile (AIBN), concentrations. Two types of cup and bob assemblies, viz., the Haake (R) SV-2 and the Haake (R) HV-DIN, have been used to measure eta(t) of the reaction mass, until reasonably high values of viscosity, well into the get effect region. Only three sets of experimental data on x (t), M,(t) and eta(t) under near-isothermal conditions, are used to develop general correlations for the viscosity. These tuned correlations predict the values of the viscosity for a whole variety of other experimental conditions, including non-isothermal cases, reflecting that the physics of the system is well represented by them. Hence, these correlations can be used for other systems after tuning their parameters. The feasibility of on-line soft sensing is demonstrated for a few cases. (c) 2006 Elsevier Ltd. All rights reserved.