A description is given of a microwave refractometer designed to measure and record the water vapor pressure in a moving air stream, particularly at very low vapor pressures. The instrument has a sensitivity of 0.004 mm Hg with a four decade range extending to 71.3 mm Hg vapor pressure. By means of microwave techniques the difference in refractive index between dry and humid air is determined and then converted to vapor pressure by an empirical formula. The instrument can be readily calibrated by the use of pure gases whose dielectric constants are precisely known. Because of the automatic null‐balancing technique used, the instrument has excellent calibration stability. Test results involving the measurement of known vapor pressures in the ranges of 0 to 0.15 mm Hg and 0 to 15 mm Hg are described. Possible applications of this instrument for the measurement of refractive index variation and its use for quality control are briefly discussed.