Abstract: The method of the determination of trace amount of thorium in the extractionprocess of thorium has been investigated. In acidic medium, chlorophosphonazo--mAreacts with thorium ion to form a purple colour complex. With ethylene glycol mo-nobutyl ether as solvent, thorium in the organic phase can be directly determined.Under the experimental conditions employed, the apparent molar absorptivity ofthorium at 670nm is 1×10~5. Beer's law is obeyed for 0--12 μg of thorium.