Low-frequency Raman scattering spectra have been measured for tert-butyl alcohol (TBA)-water and tetrahydrofuran (THF)-water binary mixtures as a function of concentration. We confirmed the isosbestic point in the reduced Raman spectra chi(')(nu) of the TBA-water and THF-water mixtures. The chi(')(nu) spectra could be decomposed into a linear combination of the neat TBA spectrum and the pure water spectrum in the frequency range from 80 to 400 cm(-1). On the other hand, below 80 cm(-1), the chi(')(nu) spectra could not be decomposed into a linear combination. The superposed spectra deviated systematically from the experimental spectra as a function of solute concentration. This result indicated that cooperative dynamics was present in TBA-water and THF-water mixtures although the intermolecular vibrations showed independent character. (C) 2003 American Institute of Physics.