Abstract: The ultrasonic method based on the dependence of speed of ultrasonic propagation ("sonic speed") on thorium concentration is used to determine thorium concentration of the organic phase in Thorex solvent extration process. The speed of ultrasonic propagation is measured by the time of ultrasonic propagation ("sonic time") in stationary distance ("sonic distance"). The parameters of temperature, thorium concentration, TBP and nitric acid concentration in TBP-kerosene system were measured as a function of the speed of ultrasonic propagation. Good linear correlations were obtained except in the case of TBP concentration. The "sonic speed" coefficients of temperature, thorium and nitric acid concentrations are-3.6 m sec-1/℃, -0.73·m·sec-1g l-1 and - 11.8·m·sec-1/N respectively. The approximate sonic speed coefficient of TBP concentration is - 0.3·m ·sec-1/1% up to 30%. The fact that the"sonic speed" coefficient of temperature is independent of thorium concentration in TBP-kerosene system is of great importance. An equation to characterize the thorium concentration measured as a function of temperature and speed of ultrasonic propagation at constant TBP and nitric acid concentrations, is shown as follows.CTh = 1706.3-1.4v-4.9(T-25),where CTh is thorium concentration, g/1; v is "sonic speed", m/sec; T is temperature, ℃.Based on the fact that "sonic speed" coefficient of temperature is independent of thorium concentration in TBP-kerosene system, a method of"dual-way comparison" is proposed to eliminate the effect of temperature for the determination of thorium concentration. The experimental results may be summarized as follows.where CThis thorium concentration in the measured channel, CThs is thorium concentration in the standard channel, A is a constant depending on "sonic distance", At is the difference of "sonic time" between measured and standard channels.The sensivity of the method, which depends on "sonic distance", is 0.3 g/1 at a "sonic distance" of 600 mm.