We have developed techniques to determine 238U, 234U and 232Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a 233U236U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that 234U 238U ratios can be measured accurately and reproducibly. 234U 238U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 109 atoms of 234U (3 × 10-8 g of total U, 10 ml of seawater). Data acquisition time is ∼ 1 hour. The small sample size, high precision and short data acquisition time are superior to α-counting techniques. 238U is measured to ± 2‰ (2σ) for a sample of 8 × 1012 atoms of 238U (∼ 3 × 10-9 g of U, 1 ml of seawater). 232Th is measured to ± 20‰ with 3 × 1011 232Th atoms (10-10 g 232Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the 232Th concentration in the oceans. Using these techniques, we have measured 238U, 234U and 232Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and 238U and 234U in vertical profiles from the Pacific. Determinations of 234U 238U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰, 2σ). The average of these 234U 238U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10-5. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is ∼ 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W). 232Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the 234U 238U ratio is consistent with the predicted range of 234U 238U using a simple two-box model and the residence time of deep water in the ocean determined from 14C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.