Beginning with Lynn and Huff [A.I.Ch.E. J. 17,475 (1971)], variable viscosity models have been applied to polymerizations in tubular reactors. Such models can become quite complex and are difficult to verify based just on the usual experimental responses of conversion, number and weight average molecular weights, and effluent temperature. Verification of predicted velocity profiles is particularly important for scale-up processes. The present study reports the validation of the velocity profile predictions using residence time measurements. The process model for the styrene polymerization is based on the steady state, two-dimensional equations ofmass, energy and momentum transport. It contains no adjustable parameters. This model predicts axial and radial velocity profiles which are then used in a tracer model. The tracer model consists of the unsteady, two-dimensional convective diffusion equation. It was solved using an apparently new technique which decouples the steps of axial convection and radial diffusion. The validity of this technique was confirmed by comparison with analytical solutions. Experimental measurements were made on tubular reactors having inside diameters of 0.011 and 0.033 m. Styrene conversions were approximately 20%. Mean residence times were 3600 s. The residence time distribution was measured using toluene as the inert tracer. Results showed excellent agreement with model predictions.