<p>The summer of 2019 was marked by an extensive early onset of surface melt and record volume losses of the Greenland Ice Sheet (GrIS), which is part of a larger trend of increasing melt over time. Given the growing spatial extent of melt, the flux of water vapor from the ice to the atmosphere is becoming an increasingly important component of the GrIS mass balance that merits investigation and quantification. We examine the isotopic composition of water vapor from Thule Air Base, NW Greenland, particularly the deuterium excess (<i>d-excess</i>), to quantify the magnitude of GrIS vapor fluxes. To do this, we observe only water vapor transported off the ice sheet (i.e., when easterly winds occur) and during the active melt season. We find that the GrIS-derived water vapor <i>d-excess</i> values are controlled by two main factors: 1) the <i>d-excess</i> of the sublimating vapor, which is determined, in part, by the relative humidity and wind speed above the ice sheet, and 2) the proportion of sublimation- vs. marine-sourced moisture. Here, the GrIS melt extent serves as a proxy for the sublimation source and the North Atlantic Oscillation provides a measure of the meridional transport of marine moisture. We demonstrate that sublimation contributes ~20 % of the water vapor transported from the GrIS during the melt season. Sublimation is thus an important component of GrIS mass balance and the regional hydrologic cycle, and this flux will become more important in the coming years as further warming continues GrIS negative mass balance trends.</p>