In synergistic extraction of metals, the partition mechanism between organic and aqueous phases of the synergist used plays an important role. Many studies on measurement of the partition equilibrium have been carried out so far, but few studies on the phase-transfer rate have been conducted. In this work, the phase-transfer rate of 5-chloro-8-hydroxyquinoline (HA) between toluene and aqueous sulfuric acid was measured using a stirred transfer cell. The transfer rate varied inversely as the first power of the hydrogen-ion concentration in a low pH range, and was proportional to the concentration of HA. Analysis of the experimental results taking into account the partition equilibrium at the interface between the organic and aqueous phases gives the mass transfer coefficients of HA in both the aqueous and organic films. The calculated results of the transfer rate using these values agree well with the experimental ones. The experimental results in the high pH range deviate slightly from the calculated ones. This tendency is also explained qualitatively by considering the effect of pH on the solubility of HA in sulfuric acid aqueous solution.