A new chemical separation technique to isolate Th and U from silicate rocks was established by using two kinds of commercial extraction Chromatographic resins. In the first column procedure, with U/TEVA·spec resin, almost all elements except Th and U were eluted by 4 M HNO3. Th was then separated by using 5 M HC1, and U was finally isolated by successive addition of 0.1 M HNO3. A significant amount of Zr still remained in the Th fraction, which was then further purified in the second column stage using TEVA·spec resin. In the second procedure, Zr was eluted first by using 2 M HNO3, and then Th was collected by 0.1 M HNO3. Both the Th and U fractions obtained by these procedures were sufficiently pure for thermal ionization mass spectrometric (TIMS) analysis. Recovery yields of Th and U exceeded 90%, and total blanks were <19 pg for Th and <10 pg for U. Our method has advantages over previous methods in terms of matrix effects, tailing problems, and degree of isolation. Since Th and U are effectively separated without suffering any matrix interference from coexisting cations and anions, this technique can be used not only for the analysis of igneous rock samples but also for the analysis of soils, marine sediments, carbonates, phosphates and seawater, groundwater, and surface water. © 1998 American Chemical Society.