A viscometer-reactor assembly has been developed and bulk polymerization of methyl methacrylate carried out in it. The viscosity of the polymerizing mixture has been measured continuously at two different temperatures (isothermal) and initiator loadings. Samples of the reaction mass have also been taken out at several different times and analyzed for monomer conversion and the weight-average molecular weight. The data obtained have been curve-fitted using the Martin equation with K-M treated as an empirical parameter. The use of experimental data on the viscosity of the reaction mass during polymerization as a state estimator (software sensor) has been demonstrated. The viscometer-reactor assembly can be used to obtain viscosity under nonisothermal conditions as well, and also to study on-line optimizing control of bulk polymerizations using appropriate on-line model-based state variable estimation and optimization software codes.