Abstract The adsorption of ethoxylated octyl phenols Triton X-100 (TX100) and Triton X-305 (TX305) and cetyltrimethylammonium bromide (CTAB) from their single and mixed aqueous solutions on silica gel and methylated silica gel at 25°C has been investigated. The adsorption from mixed solutions of CTAB-TX100 or CTAB-TX305 on silica gel exhibits a synergistic effect at low concentrations and an antagonistic effect at high concentrations, but the effects are less pronounced for the latter system. It is suggested that the synergistic effect is due to the additional interaction between the hydrocarbon chains of adsorbed TX molecules and adsorbed CTA + ions. The antagonistic effect, which occurs at high concentrations, is believed to result from the formation of mixed micelles in the bulk solution. In contrast with that on the silica gel, the adsorption from mixed solution of TX100-CTAB on methylated silica gel shows no synergistic effect at low concentrations since the CTA + ion is not adsorbed. At high concentrations, although the adsorption of TX100 is obviously depressed in the presence of CTAB, the adsorption of CTA + ion becomes significant in the presence of TX100. It is suggested, as above, that the decrease in the adsorption of TX100 is due to the formation of TX100-CTA + mixed micelles in the bulk solution. At high concentrations of TX100, some CTA + ions may be incorporated into the adsorbed TX100 layer by the hydrophobic interaction between neighbouring hydrophobes (surface solubilization). However, the results show that neither TX305 or CTAB are adsorbed onto methylated silica gel either from their respective single solution or from their mixed solution.