Adsorption of tetraalkyl ammonium hydroxide (TAAH) with mesoporous silica such as MCM-41 and MCM-48 has been investigated. The adsorption of TAAH with the mesoporous silica increases with increase in pH of the aqueous solution. The maximum adsorption amounts of TAAH with the mesoporous silica, determined by Langmuir isotherms, correspond to the relationship between the pore diameter of the mesoporous silica and the molecular size of TAAH, while no significant relationship is observed with both the specific surface area and the amount of silanol group of the mesoporous silica. Mesoporous silica having large pore diameter is applicable to the adsorptive separation of TAAH from the effluent solution.