The biosorption characteristics of uranium(VI) Onto Catenella repens (a red alga). were evaluated as a function of pH, biosorbent size, time, biomass dosage, initial uranium concentration and temperature. Within the pH range Studied (1.5-7.5), 4.5 was the optimum pH for the uptake of uranium(VI) by C repens. Reduction in particle size did not increase the biosorption capacity. The metal removal was rapid. with more than 90% of total biosorption taking place in 30 min. and equilibrium was attained in 45 min. The maximum metal loading capacity of the alga was 303 mg/g. Within the temperature range studied (15-55 degrees C), there was no significant change in biosorption, under optimal conditions. Adsorption process could be well defined by both the Langmuir and Freundlich isotherms with 1 2 of 0.94 and 0.96. respectively. The kinetic data fitted the pseudo-second-order kinetic model with the 1 2 Value of 0.99. At a low pH of 2.5. where most of the biomasses show either no or less metal uptake. a good (> 15%) metal loading capacity of 25% was achieved. Therefore biosorption characteristics were also evaluated at pH 2.5. (C) 2008 Published by Elsevier B.V.