Real-time implementation of a robust multirate state estimator on a continuous stirred-tank, free-radical, styrene polymerization reactor is presented. The estimator is designed based on a first-principles model of the process. It has integral action that allows for robust estimation of the process states. The estimator calculates frequent robust estimates of polymer weight-average and number-average molecular weights and monomer concentration. These are estimated from frequent online measurements of the reactor temperature, and infrequent offline and delayed measurements of polymer weight-average and number-average molecular weights and monomer concentration. Infrequent measurements of monomer concentration are obtained by gas chromatography and gravimetry, and those of weight-average and number-average molecular weights by gel permeation chromatography.