Experimental results are given for the absorption coefficient of atmospheric water vapor in the longwave region of the submillimeter range. The measurements were made under natural conditions by the varying humidity and varying distance methods, with a monochromatic source of radiation. The smallest values of absorption occurred in the range =1–1.15 mm; here under normal atmospheric conditions (p=760 mm Hg, T=293° K, ₀=7.5 g/m³), varied from 5.5–3.5 dB/km. In the other relative transparency window, =0.87 mm, the absorption coefficient was equal to 10.8 dB/km. The experimental results are compared with theoretical calculations.