In batch reactor operation, a reliable method of knowing the reaction state on-line is an important requirement for quality regulation between batches and minimum batch cycle time. An on-line quality control scheme using conductance measurement of the reaction mixture is presented for a pre-defined optimized operation of a batch esterification reactor. Through a series of experimental studies, a semi-empirical equation for the conductance-conversion correlation is derived. Using this conductance measurement together with a reaction model, a cascade nonlinear control scheme for the batch esterification reactor is designed. Additionally, to solve the problem of model inaccuracies and uncertainties, the extended Kalman filter is applied for the reaction state estimation. Experimental studies have been conducted to illustrate the performance of the control scheme. The proposed approach shows better control performance than the commonly used temperature control approach in controlling the batch esterification reactor.