Understanding what controls the water vapor isotopic composition of the sub-cloud layer (SCL) over tropical oceans (δD 0) is a first step towards understanding the water vapor isotopic composition everywhere in the tropo-sphere. We propose an analytical model to predict δD 0 motivated by the hypothesis that the altitude from which the free tropospheric air originates (z orig) is an important factor: when the air mixing into the SCL is lower in altitude, it is generally moister, and thus it depletes the SCL more efficiently. We extend previous simple box models of the SCL by prescribing the shape of δD vertical profiles as a function of humidity profiles and by accounting for rain evaporation and horizontal advection effects. The model relies on the assumption that δD profiles are steeper than mixing lines, and that the SCL is at steady state, restricting its applications to timescales longer than daily. In the model, δD 0 is expressed as a function of z orig , humidity and temperature profiles, surface conditions, a parameter describing the steepness of the δD vertical gradient, and a few parameters describing rain evaporation and horizontal advection effects. We show that δD 0 does not depend on the intensity of entrainment, in contrast to several previous studies that had hoped that δD 0 measurements could help estimate this quantity. Based on an isotope-enabled general circulation model simulation, we show that δD 0 variations are mainly controlled by mid-tropospheric depletion and rain evaporation in ascending regions and by sea surface temperature and z orig in subsiding regions. In turn, could δD 0 measurements help estimate z orig and thus discriminate between different mixing processes? For such isotope-based estimates of z orig to be useful, we would need a precision of a few hundred meters in deep convective regions and smaller than 20 m in stratocumu-lus regions. To reach this target, we would need daily measurements of δD in the mid-troposphere and accurate measurements of δD 0 (accuracy down to 0.1 ‰ in the case of stratocumulus clouds, which is currently difficult to obtain). We would also need information on the horizontal distribution of δD to account for horizontal advection effects, and full δD profiles to quantify the uncertainty associated with the assumed shape for δD profiles. Finally, rain evaporation is an issue in all regimes, even in stratocumulus clouds. Innovative techniques would need to be developed to quantify this effect from observations.