Water vapor is of considerable interest as a product of combustion and more recently as a constituent of the fuel itself, such as in syngas. Through the use of spectroscopy and the application of the Beer-Lambert Law, the concentration of a given species, in this case water vapor, can be determined. A tunable diode laser has been sent through a sample of water vapor. Depending on the wavelength of the laser, the absorption coefficient at that wavelength, and the pathlength of the sample, the concentration of water vapor can be determined. This process can also be extended through the use of two lasers emitting at different wavelengths to determine the temperature, pressure, and velocity of the measured species in the sample. It can also be used on other species of interest, such as carbon dioxide, nitric oxide, or the oxygen molecule.