It is important to elucidate the effect of the support on retention when investigating the retention mechanism in reversed phase liquid chromatography. The objective of this research was to examine how octadecyl-modified phase can control the unique nature of Titania. Titania was synthesized in our laboratory by the sol-gel method. Octadecyl-modified Titania was prepared by silylation with a octadecyltriethoxysilane. By using anionic, cationic. acidic and basic solutes, the retention behaviours of the Titania and the octadecyl-modified Titania were determined under reversed phase conditions. In the use of aqueous methanol mobile phases containing acetic acid-sodium acetate or bicine-sodium hydroxide buffer, we observed that the Titania synthesized behaved as an amphoteric ion exchanger, and that the retention behaviours of the octadecyl-modified Titania were significantly influenced by octadecyl-modification. The effect of octadecyl-modification on retention was ascribed not only to octadecyl groups but also silanol groups, which were formed by silylation. The retention behaviour as a whole, however, was clearly based on behaviour as an amphoteric ion-exchanger. Thus, we concluded that the octadecyl-modified phase, although it really influenced the retention behaviour, could not accomplish controlling the amphoteric ion exchange ability of the Titania in these experiments.