The ability of HCl-water hyacinth stems (HCl-WHS) biomass powder to adsorb acid red 73 (AR73) dye, from aqueous solutions has been investigated through batch experiments. The dye removal was found to be dependent on adsorbent dosage, initial dye concentration, mixing speed, contact time and adsorbent particle size. All batch experiments were carried out at constant pH and temperarure of 3.0 and 25°C respectively. Pseudo-first-order and pseudo-second-order models have been used to represent the process kinetics and obtain the main kinetic parameters. The results show that the pseudo-first order model is the one that best describes the biosorption of the AR73. The experimental isotherm were analyzed using the Langmuir and Freundlich equations. The maximum adsorption capacity was found 50 mg AR73 per g HCl-WHS powder. The thermodynamic study indicated that the adsorption of AR73 onto HCl-WHS surface was favorable, spontaneous and endothermic.