A sensor for simultaneous and continuous 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. Laboratory calibrations of density and temperature were acquired in a variable temperature absorption cell and on a H₂-air flat flame burner from 500 to 2200 K. Laboratory determinations showed absolute accuracies of 5% in temperature and 10% in molecular density for a 0.3 Hz bandwidth. An optical system was developed for integrating the sensor to a model SCRAMJET combustor at the Air Force Research Laboratory Wright-Patterson Air Force Base. Preliminary measurements in the Mach 2.1 flow field at expanded temperatures of 540, 650, and 740 K across an 18 cm pathlength resulted in temperature measurements that agreed with theoretical predictions to within 7 to 11%. Density measurements agreed with predictions to within 17 to 56%. Velocity measurement precision was compromised by beam steering effects, although results in reasonable agreement with predictions were observed at the lowest enthalpy condition.