Abstract: Biosorption kinetics of lead (II) and cadmium (II) from aqueous solutions by Sargassum sp. biomass were studied. Kinetic experiments were done in three initial concentrations of Pb2+ and Cd2+ and fixed initial ratio of adsorbate to biosorbent in single component batch system. The heavy metals uptake was found to be rapid for all concentrations and reached to 88-96% of equilibrium capacity of biosorption in 15 min. Monitoring of pH in the kinetic experiments presented the simultaneous release of H+ with the uptake of Pb2+ and Cd2+, therefore ion exchange was confirmed to be one of the biosorption mechanisms. The experimental data were analyzed using four sorption kinetic models; the pseudo first-order (Langergren), pseudo second-order, saturation (mixed-order) and second-order rate equations. The pseudo second-order and saturation rate equations described the biosorption kinetics of Pb2+ and Cd2+ with high correlation coefficient (R2>0.99) and better than the other equations. Kinetic analysis presented a direct relationship between initial Pb2+ and Cd2+ concentrations and the rate of biosorption. The maximum uptake capacities of Sargassum sp. biomass for Pb2+ and Cd2+ were obtained 1.07 and 0.79 mM g-1, respectively.