We are building a portable high speed, accurate, spectroscopically-based water vapor (relative humidity) monitor. It is capable of measuring water vapor with a noise level of <300 ppm (± 1% relative humidity) in a one second sampling period with a 0.60 meter path length. Water vapor concentrations are measured using the optical absorption of a Zeeman-split emission line of an argon discharge lamp. When a longitudinal magnetic field is used to split the emission line, two components of opposite circular polarization are produced: one coincident and one off-resonance with respect to a water vapor absorption line. Differential absorption is directly measured by alternately selecting the polarization of the light which is allowed to propagate down the optical path into an absorption cell. The instrument acts as a mixing ratio monitor (i.e., measurements are independent of pressure) over a pressure range of 0.3 to 1.2 bar.