Abstract: Gases in the atmosphere are detected remotely using a frequency-mixed CO.sub.2 laser beam formed by passing the beam from a first CO.sub.2 laser through a frequency doubler and then frequency-adding the output to the frequency from a second CO.sub.2 laser to obtain wavelengths in the 3 micron region. A first wavelength in this region, preselected for nonabsorption by the gases to be detected, is transmitted through the gases toward an object capable of reflecting the beam back. A second wavelength preselected as highly absorbed by the gases to be detected is then transmitted. The presence and quantity of the gases is then determined by the difference in the amount respectively absorbed at the two wavelengths.