This paper reports a new method for the spectrophotometric determination of trace thorium with chlorophosphonazo-mA and cetylpyridium bromide (CPB).In dilute formic acid medium at pH 1-3 thorium forms a blue-violet colour ternary complex with the above mentioned reagent. The complex exhi- bits maximum- absorption at 680 nm and the value of molar absorption coeffi- cient is found to be 1.41 × 105. The concentration of thorium obeys Beer's law in the range of 0-25 ug/25 ml. When 743 cation exchange resin is used a 5% sodium citrate-0.05% EDTA solution is recommended as new eluant for separation and concentration of tho- tiuav.. The precision is better than ±4.79%.