Two seaweeds (Sargassum muticum, S.m. and Gracilaria caudata, G.c.) collected on the coasts of Cuba have been tested and compared for nickel biosorption. The metal was efficiently bound to the biomass at pH 3 for Sm. and pH 5 for Cc. Sorption isotherms, at the optimum pH, showed that Sm. is more efficient than G.c.: maximum sorption capacity reached about 70 mg Ni g(-1) and 45 mg Ni g(-1) for S.m. and Cc., respectively. The isotherms were modeled using the Langmuir equation (which fits better experimental data than the Freundlich and Temkin equations). Sorption kinetics were also carried out varying metal concentration, sorbent dosage particle size and temperature. The kinetics were modeled using the pseudo-second order rate equation and the intraparticle diffusion equation. (C) 2010 Elsevier B.V. All rights reserved.