A chelation column preconcentration method has been developed for the determination of uranium and thorium in waters by ICP-MS. Detection limits of 24 pg and 60 pg respectively were obtained, but these were blank limited. Uranium and Thorium were determined in certified reference materials. Results for uranium were 121 ± 21 and 15 ± 3 ng g-1 in NIST 1566a and NIST 1575 compared with certified values of 132 ± 12 and 20 ± 4 ng g-1 respectively. Results for thorium were 29 ± 8 and 28 ± 5 ng g-1 in NIST 1566a and NIST 1575 compared with indicative and certified values of 40 and 37 ± 3 ng g-1 respectively. The on-line separation of actinide radionuclides was achieved by selective elution of U, Th, Pu, Np, and Am.