Tailored polymer resins are frequently required for a given application. The lack of instruments for in-line monitoring of polymer quality has long been recognized as an important problem in polymerization reactor control. Using the styrene solution polymerization system as an example, we present the use of near-infrared (NIR) spectroscopy as an alternative tool for in-line and in situ monitoring and control of monomer conversion and average molecular weight of polymer resins. By using a Kalman filter state estimator and an accurate first-principle model, the control loop could be successfully closed to track desired monomer values and average molecular weights. Two process control strategies, one based on the optimal control theory and the other on model predictive control, were implemented both theoretically and experimentally. The experimental results showed that it is feasible to use NIR spectroscopy for the simultaneous control of monomer conversion and polymer average molecular weight. (C) 2003 Wiley Periodicals, Inc.