The stratospheric H2O mixing ratio profile in the 10-24 km altitude range is derived from in-situ measurements, using a new technique: OH fluorescence produced when H2O molecules are photodissociated by a V.U.V light source. The flight instrument and its calibration are described in some details. A decrease of the mixing ratio is observed below 24 km, down to 21 km. The stratosphere-troposphere transition shows vertical structure.