Abstract: The colour reaction of Th with 2-(2-arsonophenylazo)-7-(2,4,6-tribromo-3-carboxylicphenlazo)-1, 8-dihydroxy-3, 6-naphthalenedisulfonic acid (Arsenazo-TBC)is investigated. The reagent and the complex of Th have absorption maxima at520 nm and 670 nm respectively. The colour reaction has high sensitivity and goodselectivity. The apparent molar absorptivity of complex of Th is 1.02×10~5l·mol~(-1).cm~(-1). Beer's law is obeyed in the concentration range of 0 to 20 μg/25 ml. Therelative standard deviation is 0.6% and the percentage recovery is 100.1%. Themethod is used for the determination of trace amounts of Th in hair and oreswith satisfactory results.