Studies of isotope hydrology of geothermal systems have generally emphasized deep circulation of meteoric water, demonstrated by constancy of deuterium contents, and an 'oxygen isotope shift' to higher oxygen 18 contents produced by water-rock reactions. However, subsurface boiling (in high-temperature systems) and dilution may cause deuterium (and oxygen 18) contents of hot spring and recharge waters to differ significantly. By using thermodynamic and isotopic fractionation data for water and steam the differences in chloride, oxygen 18, and deuterium between surface and deep thermal waters have been calculated for 'single-stage' (in which steam remains mixed with water and separates near a single temperature) and 'continuous' (in which steam is separated as it is formed) steam separation. Although changes in chloride concentration resulting from these processes are nearly identical, isotopic changes differ greatly. Single-stage steam separation from 250¿ to 95¿C results in increases in Cl, 103 ln Δ18O, and 103 ln ΔD(Δ=<(103+Δsurf)/ (103+Δdeep)>) of 1.44 times, 1.75 0/00, and 9.1 0/00, respectively, whereas continuous steam separation results in increases of 1.41 times, 1.05 0/00, and 3.1 0/00. Because of these effects, deuterium contents of hot springs may be substantially higher than those of recharging waters, and part of the observed oxygen isotope shift may be due to boiling. These calculations have been applied to the deuterium and chloride compositions of hot spring and shallow drill hole waters of Norris, Lower, and Shoshone Geyser basins in Yellowstone National Park, Wyoming. The diverse compositions observed could result from boiling with 'multiple-stage' (intermediate between single-stage and continuous) steam separation of mixtures of a single deep thermal water (360¿C, 310 ppm Cl, and -149 0/00 ΔD) with cold dilute (5¿C, 2 ppm Cl) meteoric waters differing in deuterium according to locality (Norris, -142 0/00; Lower, -144 0/00; and Shoshone, -133 0/00). The recharge to the deep thermal water must be precipitation remote from the geyser basins or ancient precipitation from a colder period.