A physical model is proposed for predicting the transport of the stable isotopes of water (H218O and HD16O) in soils under natural climatic conditions. The transport equation for stable isotopes in the unsaturated zone was coupled with a mechanistic model for water and heat exchange across the soil-atmosphere boundary. Diffusive and convective isotope fluxes were each considered in liquid and vapor phases of the soil. Turbulent exchanges of isotopes between the soil surface and the atmosphere were modeled by use of similarity theory in the same way as water vapor. Boundary conditions at the soil surface are based on an energy balance and turbulent fluxes of heat, water, and isotopes. The resulting model provides a framework for understanding the behavior of stable isotopes in soils of variable hydraulic properties and under various climatic conditions. ¿ American Geophysical Union 1996