Mass transport accompanying the redox process of ferrocenyle-undecanethiol monolayers has been examined by an electrochemical quartz crystal microbalance (EQCM) for both compact and loosely packed layers with different surface coverages prepared by self-assembly method. Ferricinium cations formed upon the oxidation of ferrocene group associate with anions in solution to form ion pairs. The anion association was not accompanied by water incorporation into the layers independent of the surface coverage, unless the layers were pretreated with water before the measurements were made. Immersion of the thiol-modified electrode in water caused a drastic increase in interfacial mass transport upon oxidation, particularly at a loosely packed layer. It is concluded that this increase is the result of the incorporation of water molecules into the space between the alkyl chains of the oriented ferrocenylundecanethiol. The space seems to be pre-occupied by hexane during the self-assembly process.