Cephalexin adsorption from aqueous solutions using SBA-15 mesoporous silica as adsorbent and the influence of pH solution on drug adsorption were studied. In order to have a better knowledge about the way the drug molecules interact with the inorganic matrix, the adsorption process was estimated by applying the computational chemistry software YAeHMOP (Yet Another extended Huckel Molecular Orbital Package). A strong correlation between the theoretical calculations and the experimental results was established, showing that the adsorbateadsorbent interaction is pH dependent. Calculated cephalexin horizontal adsorption energy was almost 9 eV more stable than the one corresponding to vertical adsorption, and also the lowest enthalpy of contact and the maximum adsorption percent were found for the cationic cephalexinsilica system. Cephalexin adsorption through the NH3+ group is 8 eV stronger than the molecule adsorption through the COO- group. In agreement with these theoretical predictions, experimental results indicate that the electrostatic attraction between CPX ions and the surface of mesoporous silica is favored at pH values between 2 and 2.56, the maximum being for cephalexin adsorption obtained at pH 2.3. (C) 2014 Elsevier B.V. All rights reserved.