This paper presents the development of a state estimator for systems with multirate measurements fixed-lag smoothing based extended Kalman filter algorithm is proposed. The performance of the algorithm has been evaluated using a case study on a complex emulsion copolymerization batch process. The proposed estimator uses measurements multiple times to achieve good convergence properties and robustness to state and measurement noise. The simulations also demonstrate that the performance of the proposed algorithm is superior to the basic extended Kalman filter.