Interface-specific observation of molecular vibrations was achieved with visible light by a fourth-order optical process. An aqueous solution of oxazine dye interfaced with air was irradiated with 16-fs pulses at 600 nm. A pump pulse excited Raman-active nuclear motions of the dye. A probe pulse interacted with the excited molecules to generate second harmonic light, the intensity of which was oscillated as a function of the pump(-)probe delay. Fourier transformation of the oscillation provided a vibrational spectrum of 0- 1000 cm(-1) for the dye located at the air-solution interface.