Abstract: A molar absorptivity of 1.65--1.85×10~5 1·mole~(-1)·cm~(-1) can be obtained withchlorophosphonazo--p--NO_2 (CPApN) as the color--forming reagent in cetylpyridini--um chloride(CPC) in the determination of micro--amount of Th in grains. Beer'slaw is obeyed in the range of 0--8 μg Th in 25 ml solution. A deviation of lessthan 5% is observed for 23 interfering ions studied, some of which, however,need the addition of masking agents such as citric acid.