This paper examines the influences of ionizable thiol monolayers on the interfacial charge-transfer rates for redox-active ions by electrochemical impedance spectroscopy. Whereas short-chain thiols such as 4-aminothiophenol hardly affected the charge-transfer resistance for Ru(NH3)(6)(3+) having a high standard heterogeneous rate constant, the 11-mercaptoundecanoic acid monolayer raised this resistance to 3 x 10(4) times that observed at a bare electrode in acidic solutions. As the terminal carboxyl group ionized, the charge-transfer resistance decreased by a factor of 300 because of a double-layer effect. When 3-mercaptopropionic acid was mixed with 1-hexadecanethiol, the charge-transfer resistance varied over four orders of magnitude with the composition of the two-component monolayer. The apparent rate constant for indigotetrasulfonate at 4-aminothiophenol-modified electrodes decreased by a factor of 25-30 as the solution pH varied from 4 to 2. This decrease was opposite to the change expected from protonation of the terminal amino group. It can be ascribed to a strong binding interaction between the redox anion and the positively charged monolayer. (C) 2003 Elsevier Ltd. All rights reserved.