A one-dimensional advective-diffusive model is developed for the evaporation process across an evaporating surface, providing a general theory of fractionation during evaporation and condensation. The model configuration is based on the Craig-Gordon linear evaporation model. We improve the model by allowing advection and diffusion to exist in the skin layer beneath the evaporating surface. The model is based on two fundamental physical principles, Henry's law and Fick's law. The model solutions show that diffusion of isotopes in both the atmosphere and the condensed phase plays an essential role in controlling the isotope flux ratio ΔF, which is a measurable quantity. There are several dimensionless parameters that control the isotopic fractionation during evaporation. The most important parameter is N, which takes into account the evaporation rate and the depth of the advective-diffusive layer and represents the relative magnitude of the rate of advection versus the rate of diffusion. Assuming rapid surface exchange, the water vapor is always in isotopic equilibrium with the liquid water at the interface. Under the slow evaporation limit, i.e., N→0, the back flux from the atmosphere to the liquid water reservoir is not negligible compared with the upward flux; hence the net flux ratio must take into account information from the atmosphere such as the mixing ratio and the isotope ratios of water vapor. When N is large, the back flux becomes negligible, so the upward flux dominates the net flux and brings only the information of the liquid water reservoir into the net flux; therefore the flux ratio approaches the isotope ratio of the liquid water reservoir. The model also justifies itself by matching with independent theoretical results from earlier studies. Vapor condensation onto ice is also included as a special case in the model. ¿ 1999 American Geophysical Union