A sensor for simultaneous measurements of water vapor density, temperature and velocity has been developed based on absorption techniques using room temperature diode lasers (InGaAsP) operating at 1.31 µm. Sensor calibrations of density and temperature were acquired in a laboratory hot absorption cell and on a H₂-air flat flame burner from 500 K to 2200 K. In the laboratory with 0.3 Hz bandwidth and a pathlength of 70 cm in the flame, the sensor provided a precision of better than 5% in temperature and better than 10% in molecular density, and velocity. Optical instrumentation was developed for integrating the sensor to a model SCRAMJET combustor at the Air Force Research Laboratory Wright-Patterson AFB. Preliminary measurements in the Mach 2.1 flowfield at expanded temperatures of 540, 650, and 740 K across an 18 cm pathlength provided a temperature agreement with predictions ranging from 7 to 11%, a density agreement ranging from 17 to 56% and a velocity agreement of 25%. Measurements at higher enthalpy flow gave similar results for water density and temperature, but generated increasingly difficult beam steering influences that degraded accurate flow velocity determinations.