Abstract: A new highly sensitive colour system for thorium is developed. An association complex of thorium ion is formed with 8-hydroxyquinoline and ethyl violet having absorption maximum at 607nm. The complex can be extracted into 10ml of a mixture of benzene and MIBK (3:1) in 1 min. Its apparent e value is 7 .5×105l·mol-14·cm-1 at 607nm.Optimum conditions for the extraction have been found to be 3.5 × 10-3mol/l sulfuric acid, 0.020mol/l 8-hydroxyquinoline and 0.018% ethyl violet. Beer's law is obeyed for 0-4.5μg of thorium per 10ml. The colour of the extract is stable in nominutes at room temperature. The effects of 32 foreign ions and 4 complex reagents on the determination are discussed.