Adsorption properties of two activated carbons were investigated as a function of several operating parameters such as initial dye concentration, contact time, pH, and temperature. Adsorption/desorption performance was related to the structural and thermodynamic properties of the adsorbent. To evaluate the effect of the adsorbate an acid dye (alizarin red S, ARS) and a basic dye (methylene blue, MB) were used. Results obtained suggest that a low basicity of the adsorbent is a key factor independent of the acid or basic nature of the adsorbate. Langmuir isotherms best represent the adsorption system. Reported maximum adsorption capacities for ARS and MB were 762 mg . g(-1) and 742 mg . g(-1), respectively. The adsorption process followed the pseudo-first-order model. The thermodynamic parameters such as Gibbs energy (Delta G degrees), entalphy (Delta H degrees), and entropy (Delta S degrees) indicate that the adsorption of the basic dye was significantly dependent on AC properties. Hydrophobic interaction played a dominant role in the adsorption of the basic dye, whereas acid dye was bound through electrostatic interaction. This coincided with the lower activation energy values reported for MB [(9.58 to 18.68) kJ . mol(-1)] in comparison to ARS [(23.79 to 27.67) kJ . mol(-1)]. Finally, for adequate desorption it is more important to enhance interaction between dye and surfactant rather than the repulsion between micelles and the adsorbent surface.