We propose to use the space-based capability of ISO + SWS to study the physical conditions in pre-main-sequence protoplanetary disks. Given its high abundance and the large number of transitions that sample a wide range in temperature, water vapor is expected to be an excellent probe of the physical properties of disks over a range of radii ( 0.1-5 AU). Although strong absorption in the Earth's atmosphere generally precludes ground-based studies of water vapor, our discovery of hot ( 2000K) water emission from pre-main-sequence disks, made in the wings of the strong telluric bands, demonstrates the viability of water as a probe of the physical conditions at small disk radii. We will use the SWS to obtain spectra of the much stronger fundamental H2O bands centered near 2.7 and 6 microns. These bands will sample cooler disk gas at larger radii (r > 0.3 AU). The data will be modeled in order to extract physical properties, gas temperature and column density, from the observed spectra.