Atmospheric water vapor was sampled simultaneously at two heights in the atmospheric surface layer above a coastal salt marsh near New Haven, Connecticut, during June 11--27, 1997. The water vapor samples were analyzed for their mixing ratio, Q, and deuterium isotope ratio, ΔD. The observed ΔD varied in the range of -145 to -89 per mil and was positively correlated with Q, reflecting the influence of in-cloud condensation and surface evapotranspiration processes at a regional scale. Both Q and ΔD were, in general, lower at the upper level, indicating an upward transport of H216O and HD16O. The Craig-Gordon model reproduced reasonably well the combined effects of equilibrium and kinetic fractionations and atmospheric demand on the evaporation process. Transpiration of the marsh plants, Spartina patens (Ait.), was a minor component (11%) of the total evapotranspiration during the experimental period. We also suggest that the relationship between ΔD and salinity, S, of the marsh surface water can be used to infer the isotope flux ratio. More data, however, are needed to verify this mass balance approach. ¿ 2001 American Geophysical Union