The water vapor absorption line resulting from the rotational transition 5-1-6-5 has been investigated experimentally. Radiation is fed into an air-filled cubical copper cavity 8 ft. on an edge. Strings of thermocouples with alternate junctions coated with a "lossy" material are placed at random in the cavity. The e.m.f. of these thermocouples is proportional to the Q of the cavity and its contents. With the total pressure inside the cavity at one atmosphere, the partial pressure of the water vapor is varied from 1 mm to 55 mm of Hg. A measurement of the change in e.m.f. with humidity yields a value for the losses in the water vapor, provided the Q of the cavity is known. This quantity may be determined from additional measurements taken with an aperture opened in the side of the cavity. The wave-length range between 0.7 cm and 1.7 cm has been explored. Results indicate a peak at ν̃=0.744±0.005 cm^-1, corresponding to a wave-length λ=1.34 cm. The absorption line is broadened as the water vapor density is increased. At very low density, the half-width of the curve (half-width at half-height) is 0.087±0.01 cm^-1, while the corresponding value for a density of 50 gram/meter3 is 0.107±0.01 cm^-1. The cross section for a water-water collision must be nearly 5 times that for a water-air collision to account for this change in half-width with vapor density. The attenuation at the peak is 0.025 db per kilometer for 1 gram of water vapor per cubic meter.