A chemical ionization mass spectrometer (CIMS) instrument has been developed for the fast, precise, and accurate measurement of water vapor (H 2 O) at low mixing ratios in the upper troposphere and lower stratosphere (UT/LS). A low-pressure flow of sample air passes through an ionization volume containing an α-particle radiation source, resulting in a cascade of ion-molecule reactions that produce hydronium ions (H 3 O + ) from ambient H 2 O. The production of H 3 O + ions from ambient H 2 O depends on pressure and flow through the ion source, which were tightly controlled in order to maintain the measurement sensitivity independent of changes in the airborne sampling environment. The instrument was calibrated every 45 min in flight by introducing a series of H 2 O mixing ratios between 0.5 and 153 parts per million (ppm, 10 −6 mol mol −1 ) generated by Pt-catalyzed oxidation of H 2 standards while overflowing the inlet with dry synthetic air. The CIMS H 2 O instrument was deployed in an unpressurized payload area aboard the NASA WB-57F high-altitude research aircraft during the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) mission in March and April 2011. The instrument performed successfully during seven flights, measuring H 2 O mixing ratios below 5 ppm in the lower stratosphere at altitudes up to 17.7 km, and as low as 3.5 ppm near the tropopause. Data were acquired at 10 Hz and reported as 1 s averages. In-flight calibrations demonstrated a typical sensitivity of 2000 Hz ppm −1 at 3 ppm with a signal to noise ratio (2 σ, 1 s) greater than 32. The total measurement uncertainty was 9 to 11%, derived from the uncertainty in the in situ calibrations.