Acid Black 1 (AB1) is a diazo dye containing both NN and CC chromophores groups that has considerable health effects being irritating for skin, eye and respiratory system. It is essential to remove this dye from wastewater. In this study, the removal of AB1 with brown macroalgae Sargassum glaucescens and Stoechospermum marginatum was studied using a batch system. The effects of parameters such as initial pH (in the range 2-10), contact time (within 90 min), biomass dosage (in the range 1-9g L-1), initial dye concentration (in the range 10-50 mg L-1), particle size of biosorbent (0.53-500 mu m) and agitation speed (from 70 to 180 rpm) on the adsorption capacity of AB1 were investigated. The equilibrium data indicated that the Freundlich model provided the best correlation of the experimental data for both biosorbents at least in the concentration range investigated in this study. The biosorption kinetic data were successfully described with pseudo-second order model (PSORE) for two biosorbents. Thermodynamic parameters showed adsorption of dye was feasible and endothermic at all temperatures (283-313 K) for both biosorbents. A decrease in particle size of biosorbents increased acidic dye removal capacity. The agitation speed influences AB1 sorption capacity and optimum agitation speed observed at 130 rpm among different agitation speed (i.e. 70-180 rpm). FT-IR analysis demonstrated the involvement of different functional groups, mainly hydroxyl, carboxyl and amine groups. These results confirm that these algae have a good potential for removal of AB1 from aqueous solution at low dye concentration. (C) 2012 Elsevier B.V. All rights reserved.