Abstract: The rate and mechanism of extraction of calcium by HDEHP in benzene from nitrate solution have been investigated using the Single Drop Method. The result indicates that the orders of reaction are 1,-1, and 1.5 with respect to [Ca²⁺], [H⁺] and [H₂A₂]_(o)respectively, and the rate of the reaction depends upon the specific surface of the droplets. The mechanism of the reaction was proposed.