The feasibility of using a recently introduced method for continuous, absolute online monitoring for step-growth polymerization was demonstrated, and polyurethane synthesis was chosen for the demonstration. The previously reported use of this method was for monitoring free-radical initiated chain-growth polymerization. The technique involves continuously withdrawing a small stream of reaction liquid from the polymerization reactor and pumping it through a series of light scattering and viscosity detectors. This permits the absolute weight-average molecular weight and reduced viscosity to be simultaneously monitored as the reaction proceeds. The final weight-average molecular weights of the polyurethane samples were compared to the separate light scattering measurements of the final products. (C) 2001 John Wiley & Sons, Inc.