The method is based on the separation of Tl(I) as Tl(2)HPMo(12)O(40), stripping of the molybdate, and measurement of the peak current in differential-pulse polarography of the molybdenum. The calibration graph is linear over the range 2-12 ppm of thallium. The relative standard deviation is 1.2% (7 replicates each containing 500 microg of thallium). The current due to reduction of the molybdenum is three times that for reduction of the equivalent amount of Tl(I) in the thallous phosphomolybdate precipitate, making the indirect approach more sensitive than direct polarographic determination of the Tl(I).