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Summary form only given. Of the many laser-based diagnostic techniques available for studying combustion processes very few are capable of direct determination of species concentration. We have previously used the powerful technique of THz time-domain spectroscopy to measure species concentration and temperature in active combustion. Here we believe we report the first direct observation of the rotational absorption lines belonging to the /spl nu//sub 2/ vibrational state of water vapor in the far-infrared region of the spectra. Such measurements have application outside of combustion spectroscopy. The ability to directly measure absorption with high sensitivity of hot samples has importance for astronomy as recent measurements of water vapor at 3300 K in sunspots has demonstrated.