Experimental data of the dynamic viscoelastic properties, storage modulus (G＇), loss modulus (G"), and phase shift (delta) as well as of the viscosity, eta, are reported for the polymerization of a free radical polymerization system (methyl methacrylate) which exhibits the Trommsdorff effect. A rheometer-reactor assembly developed in our laboratory is used for this purpose. It is observed that in the early stages of reaction, data lie in the terminal zone. As the polymerization progresses, the Trommsdorff effect leads to a sharp increase in both the polymer concentration and the weight average molecular weight, and the viscoelastic properties then lie in the entanglement zone. A modulus crossover point:(when G＇ = G") is identified that could be used as an identification of the point where the Trommsdorff effect starts assuming significance.