Adsorption of azo reactive dyes by metal hydroxide sludge were investigated using CI Reactive Red 2 (RR-2), CI Reactive Red 120, (RR-120), and CI Reactive Red 141 (RR-141). The adsorption isotherms, including the Langmuir constants (Qdegrees and b) and the Freundlich constant (K-f), for RR-2 decreased with increasing temperature, but this was reversed for RR-120 and RR-141. This behavior implied an exothermic process for RR-2 but an endothermic process for RR-120 and RR-141. The enthalpy value of adsorption for RR-2, RR-120, and RR-141 was -5.56, 2.77, and 6.41 kJ/mol, respectively, indicating that the adsorption of the less charged dyes (RR-2) was mainly physical, but that of the more charged dyes (RR-120 and RR-141) was chemical. The optimum system pH of 8.6 +/- 0.3 was maintained even when the solution pH was varied from 3 to 10. Higher concentration or more valence of anions of electrolytes in dye solution caused decreasing dye adsorption efficiency of metal hydroxide sludge. A higher dosage of sludge is required for real textile wastewater (>1% w/v) than for the synthetic dye solution (0.2% w/v). The leachates of heavy metals from metal hydroxide sludge to the environment are very low, which are within the standard limit of industrial effluent and leachable substances. (C) 2003 Elsevier Inc. All rights reserved.