Water wettability of Si/SiO2 surfaces was controlled by the formation of SAMs terminating in 1-alkyl-3-(3-silylpropyl)imidazolium ions and the anion exchange on the surfaces ("direct anion exchange"). The exchange was confirmed by X-ray photoelectron spectroscopy, and the water wettability was measured as a water contact angle by contact angle goniometry. We found that anions played a great role in determining water wettability of Si/SiO2 surfaces. For example, water contact angles of Si/SiO 2 surfaces presenting 1-methyl-3-(3-silylpropyl)imidazolium ions changed from 28 to 42° when the counteranion Cl- was exchanged with PF6-. In addition to the anions, the N-alkyl groups of imidazolium cations were also found to be important in determining water wettability: we did not observe any significant changes in the contact angles of Si/SiO2 surfaces presenting 1-butyl-3-(3-silylprppyl)imidazolium ions by the anion exchange. We also demonstrated that the reaction rate of the direct anion exchange was affected by a choice of solvents: the anion exchange from Cl- to PF6- was the fastest in an aqueous solution.