The solubility behaviour of uranium(VI) has been studied in a number of waters representative of the pore waters to be expected in candidate repository backfill concretes and also in solutions of calcium and sodium hydroxides. The sorption of uranium has been studied on one of these candidates, a Portland cement with a slag filler. The same cement has been hydrothermally treated to simulate the temperature conditions in the repository and the product used for comparative studies.The paper describes the experimental measurements and the results. The aqueous concentration of uranium over the uranates formed in the presence of sodium and calcium hydroxides was constant above pH 7 at 3 × 10−6 M. In the concrete pore waters the solubility of uranium was equal to or lower than this value. The distribution coefficient, Rd, for the sorption of uranium onto the slag cement was 2.5 × 104 ml g−1, and was increased by a factor two after hydrothermal treatment.