Adsorption of two organic dyes (alizarin red S and phenol red) on mesoporous silica and hybrid gels is investigated. The mesoporous gels are derived from tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), vinyltriethoxysilane (WES), propyltriethoxysilane (PTES), and phenyltriethoxysilane (PhTES). The experimental results demonstrate that the adsorption capacity of the hybrid gels is much higher than that of the pure silica gel and increases as the gel surface becomes more hydrophobic. This suggests that the organic dye adsorption is governed mainly by the hydrophobic interaction between the organic dyes and the gel surface. Langmuir and Toth isotherm models are also tested against the experimental isotherm data. (C) 2004 Elsevier B.V. All rights reserved.