A molecular weight distribution (MWD) estimator for batch methyl methacrylate solution polymerization is implemented experimentally for on-line control and estimation of the MWD. The estimator is based on an extended Kalman filter and provides current estimates of the entire MWD, reaction temperature, monomer conversion, and initiator conversion. It uses a detailed polymerization model, on-line monomer conversion measurements, temperature measurements, and periodic, time-delayed measurements of the MWD from an on-line size-exclusion chromatograph. The estimator is shown to perform well with several on-line MWD estimation experiments. Real-time feedback control of the molecular weight is presented by utilizing the on-line MWD information. Temperature, monomer-addition, and simultaneous temperature and monomer-addition control are investigated experimentally to achieve a specific constant weight-average molecular weight. The on-line feedback control is effective in rejecting realistic disturbances which deteriorate molecular weight control.