The adsorption behavior of two reactive dyes (Reactive Red 23 and Reactive Blue 171) and two acid dyes (Acid Black 1 and Acid Blue193) from aqueous solution on fly ash was investigated in order to identify the ability of this waste-material to remove colored textile dyes from wastewater. For this purpose a series of batch tests were carried out as a function of solution pH value, contact time, dye concentration and adsorption temperature. The experimental findings showed that the removal of four dyes on fly ash was a pH-dependent process with the maximum adsorption capacity at the initial solution pH of 7.5-8.5 for reactive dyes and 5-6 for acid dyes. Adsorption equilibriums of each anionic dye on fly ash could be reached within 60 min at respective optimum pH at 293 K. An increase in the initial dye concentration enhanced the adsorption capacity, but failed to increase the dye removal efficiency. The adsorption capacity for Reactive Red 23, Reactive Blue 171, Acid Blue193 and Acid Black 1 was found to be 2.102, 1.860, 10.937 and 10.331 mg g(-1), respectively. Kinetic studies of four dyes followed the pseudo-second-order modal. Freundlich isotherm described the equilibrium data of acid dyes on fly ash better than Langmuir isotherm, but Langmuir isotherm showed better fit to the equilibrium data of reactive dyes. Different thermodynamic parameters such as the free energy, enthalpy and entropy of adsorption of the dye-fly ash systems were evaluated and it was found that the reaction was spontaneous and endothermic in nature. (C) 2010 Elsevier B.V. All rights reserved.