Compact airborne humidity sensing devices using capacitive sensors are employed on board in-service aircraft to measure water vapor concentrations in the troposphere up to 13 km altitude. The sensors are individually calibrated before onboard installation. After every 500 flight hours, each sensor is calibrated in an environmental simulation chamber under typical middle/upper tropospheric flight conditions. A Lyman-Alpha fluorescence hygrometer is used as reference instrument. Preflight and postflight calibration of each flown sensor agreed very well and showed good response. Typical overall uncertainties for the 1995 Measurement of Ozone by AIRBUS In-Service Aircraft (MOZAIC) relative humidity (RH) measurements are within ¿4% RH in the middle troposphere, increasing to ¿7% RH between 9 and 13 km. In-flight comparison of the MOZAIC humidity device with other water vapor measuring techniques showed agreement within ¿(5--10)% RH and a time response of better than 10 s in the lower/middle troposphere, increasing to values of 1--3 min at 10--12 km altitude. ¿ 1998 American Geophysical Union