The surface-sensitive spectroscopic technique of optical second-harmonic generation (SHG) is applied to the in situ study of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant SHG form molecules which exhibit a large non-linear optical response at a specific wavelength can be used to measure the relative surface coverage of surfactants at the ITIES as a function of the external electro-chemical parameters. In addition, the polarization dependence of the resonant surface SHG can be used to estimate the average molecular orientation of adsorbates at the liquid/liquid electrochemical interface. As an example, the adsorption of the surfactant 4-(4′-dodecyloxyazobenzene)benzoic acid at the water/1,2-dichloroethane interface is characterized as a function of applied potential, surfactant concentration and aqueous pH with in situ resonant molecular SHG measurements. An analysis of the SHG data results in the determination of the local potential and surface pH experienced by the surfactant.