This work presents a mathematical model for semibatch carboxylated styrene/butadiene (XSBR) emulsion polymerizations, intended for the online and real-time data reconciliation and monitoring of the polymerization reactor. Proposed procedures assume that some parameters must be estimated in real time, for accommodation of the unavoidable fluctuations of industrial operation conditions. Pressure, temperature, and feed rate profiles are used for parameter estimation, allowing for real-time prediction of important properties of XSBR latexes, including monomer conversions, solids contents, and copolymer compositions. The proposed scheme was validated with actual data obtained in Pilot-plant and full-scale reactors. In all cases, estimated (n) over bar values ranged from 0.1 to 1.7, which agree with results reported previously. As observed at plant, heat transfer coefficients can change from batch-to-batch and experience large changes during the batch due to modification of the reactor volume and accumulation of polymer material on the heat transfer areas. POLYM. ENG. SCI., 51:1919-1932, 2011. (C) 2011 Society of Plastics Engineers