Alkali metal ion recognition with [2-hydroxy-5-(4-nitroplieiiylazo)phenyl]-methyl-15-crown-5 (azoprobe 1) at the heptane/water interface was investigated by in situ second harmonic generation (SHG) spectroscopy. Upon addition of alkali metal ions, the second harmonic (SH) intensity of azoprobe I at the heptane/water interface was found to increase selectively. The observed selectivity of K+ > Na+ > Li+ > TMA(+) was essentially the same order as that of the extractability for alkali metal ions in the 1,2-dichloroethane/water extraction. Although the Li+ extractability by azoprobe 1 was very low in die 1,2-dichloroethane/water system, the observed increase of SH intensity suggested that the Li+ complex could distribute to sonic extent at the heptane/water interface. The red shift of the SHG spectra for azoprobe 1 revealed that the heptane/water interface had an intermediate polarity between those of the two solvents. The light polarization analysis of the SH intensity exhibited a clear orientation change of azoprobe 1 at the heptane/water interface on forming the alkali metal complexes. It was experimentally clarified that the Na+ and K+ complexes were flatter while the Li+ complex exhibited a lift-up orientation as compared with the free anionic azoprobe 1 at the heptane/water interface.