The transfer of Rose Bengal dianion (RB2-), a fluorescent ion with asymmetric charge distribution, across the 1,2-dichloroethane (DCE)\ water (W) interface, has been studied using ac-modulated voltfluorometry, a method that combines ac voltammetry with fluorometry. The phase angle of the ac component of the fluorescence shifts to a value that cannot be explained by the theory based on the simple combination of the heterogeneous ion transfer and mass transfer. This anomaly strongly suggests that RB2- ions transiently adsorb at the interface in the course of the heterogeneous ion transfer. The rate-determining step seems to be the adsorption-desorption of RB2- ions between DCE and the interface. The apparent standard rate constant for ion transfer across the interface is estimated to be 1.4 x 10(-2) cm s(-1). The adsorption of RB2-appears to decelerate the rate of ion transfer across the interface.