We report on the development of an in-situ instrument to measure the isotopic composition of water vapor in the upper troposphere and lower stratosphere from high-altitude aircraft. Water vapor isotopic composition is a critical tracer for understanding the transport and origin of upper tropospheric water vapor and the controls on stratospheric humidity. However, measurement of water vapor isotopic composition in this altitude region is difficult because of the extreme dryness of the air and the low concentrations of the heavy isotopologues (< 1 ppb for HDO). The instrument described here utilizes the relatively new technique of integrated cavity output spectroscopy (ICOS) to increase sensitivity over more traditional tunable diode laser absorption spectroscopy. Light from a quantum cascade semiconductor laser is directed into an optical cavity consisting of a pair of highly reflective mirrors (R > 99.99%) and tuned over the wavelength region of interest; the transmission through the cavity provides an absorption spectrum with a pathlength approaching 10 km. Expected signal-to-noise for the rare isotopologues is greater than 10 for 1-s data across the tropopause. The instrument is designed to obtain measurements of H₂O, HDO, H₂¹⁸O, H₂¹⁷O, and CH₄ in a single scan. In flight, the instrument is used with a selective-screening inlet to exclude ice particles and minimize wall interactions. We will discuss issues involved with water vapor sampling in a closed cell and strategies taken to minimize wall effects and ensure accurate calibration. Science flights with the instrument will be conducted from NASA's WB-57F aircraft in 2003.