Thermal phase behavior of triethylamine (TEA)-water mixtures with various concentrations has been studied by use of near-infrared (NIR) spectroscopy. Bands due to the first overtones of C-H stretching bands (6000-5500 cm(-1)) of TEA in the NIR spectra of one-phase region of the mixtures show significant shifts to a lower wavenumbers during heating process. It has been revealed that the magnitude of the band shift for the critical mixture (32.1 wt%) is markedly prominent in comparison with that of the off-critical mixtures. Since the C-H band shifts to the lower wavenumber are accompanied by the condensation of TEA, these results suggest that the microscopic association of non-polar moieties acts as a trigger for the LCST-type phase separation. (C) 2004 Elsevier B.V. All rights reserved.