Report describing a gravimetric hygrometer that serves as the NBS standard instrument for the measurement of the moisture content of gases on an absolute basis in terms of mixing ratio (mass water vapor/ unit mass of dry gas). The National Bureau of Standards and other laboratory reference and working instruments are compared and calibrated with this instrument. The measuring operation involves the absorption of the water vapor from a water vapor-gas mixture by a solid desiccant and the determination of the mass of this water vapor by precision weighing; it also involves the determination of the volume of the associated gas of known density by counting the fillings of two calibrated stainless steel cylinders. The construction and operation of the instrument is described. Discussions of the tests and calibrations of component parts, and of the sources of errors also are included. There is also an analysis of the random and systematic errors effecting the overall accuracy in the determination of mixing ratio.