In the preparation of KH2PO4 using the extraction method, Cl- in the aqueous phase could react with the H3PO4-TOA complex in the organic phase. This paper studied the kinetics of this ion-exchange reaction by conducting the reaction between the KCl aqueous phase and the organic phase of H3PO4/TOA in n-octanol in a microchannel device. Extraction equilibria of the mixed solutions of H3PO4 and KCl by TOA in n-octanol were investigated. And equations were obtained to calculate the equilibrium constants of HCl and H3PO4 extraction by TOA, respectively, in the temperature range from 289.7 to 330.2 K. The experiments of kinetics were performed with the aqueous phase of KCl solution and the oil phase of H3PO4 loaded TOA in n-octanol, and a kinetic model was established to describe the reaction-extraction process in the microfluidics. The forward and backward rate constants for the ion-exchange reaction were determined. Apparent activation energy of 31.2 kJ/mol for the forward reaction and 27.7 kJ/mol for the backward reaction was obtained. Furthermore, using the obtained rate constants, we successfully predicted the selective extraction behavior of HCl when the ion-exchange reaction is involved. This work could be helpful for the design and optimization of KH2PO4 preparation using the extraction method. (C) 2015 Elsevier B.V. All rights reserved.