This paper describes an experimental investigation of the 1.63‐mm H2O absorption line for nitrogen‐water‐vapor mixtures and for oxygen‐water‐vapor mixtures. Frequency sweep measurements were made at pressures between 1 and 5 mm of mercury and measurements at discrete frequencies were made for pressures between 200 and 760 mm of mercury. These measurements were made over a range of temperatures from 242° to 325°K. Data for the attenuation and line breadth constant as a function of temperature ar