Construction of molecular recognition systems at solid/liquid and liquid/liquid interfaces is one of a current topic in host/guest chemistry, because the interfacial regions possess unique physical and structural properties. In this article, we describe an application of second harmonic generation (SHG) spectroscopy to observe the process of molecular recognition at solid/liquid and liquid/liquid interfacial regions. A surface specific complexation reaction of a thiourea-based receptor monolayer with inorganic anions was confirmed at the gold/acetonitrile interface by analyzing second harmonic (SH) intensity as a function of bulk anion concentrations. A molecular association of rhodamine dyes at the heptane/water and hydrophobized-silica/water interfaces was observed by SHG spectra. An analysis of polarization dependent SH signal revealed that a complexation of azoprobe with alkali metal cations caused a change in the molecular orientation at the heptane/water interface.