The design and enlargement of extraction equipment require kinetic data on the extraction system. In this study, the extraction kinetics of phosphoric acid from the phosphoric acidcalcium chloride solution by tri-n-butyl phosphate (TBP) was investigated using a constant-interfacial-area cell with laminar flow. The extracted complexes were demonstrated to be H3PO4 center dot TBP, H3PO4 center dot 2TBP, and H3PO4 center dot 3TBP by equilibrium studies. The effects of stirring speed, temperature, and interfacial area on the extraction rate were investigated. The results indicate that the extraction process is under mixed control by the extraction reaction at the interface and the diffusion of the reactant in the aqueous phase. The extraction rate is first-order with respect to H3PO4 and TBP with a rate constant of 10(-5.34) L center dot mol(-1) center dot s(-1). A mixed-control regime model is proposed. The forward extraction rate constant and the diffusion coefficient of the reactant in the aqueous phase were calculated to be 6.86 x 10(-6) and 6.47 x 10(-5) L center dot mol(-1)center dot s(-1), respectively.