Subtropical humidity plays a critical role in the radiative balance of the planet, and there is a need for adequate description of the controls on water vapor distributions. This study tests whether an advection-condensation model, combined with Rayleigh distillation, can describe observed humidity and water vapor isotope ratios of the subtropical free troposphere. A field campaign, from 9 October to 6 November, 2008, included continuous in situ measurement of water vapor stable isotope ratios at the NOAA Mauna Loa Observatory (MLO), Hawaii. Last saturation patterns for air at the MLO were determined using both Lagrangian back-trajectory and Eulerian model techniques. Last saturation occurs primarily along midlatitude storm tracks (∼65%), and secondarily near Hawaii (∼10%) within mesoscale convective systems. Periods of lower δD values at MLO correspond to extra-tropical last saturation, while elevated δD corresponds with saturation locations near Hawaii. To a first order, the conditions of last saturation are found to set not only the humidity but also the water vapor isotope ratio. In the absence of mixing, reconstructed q and δD values underestimate the observations. Experimental reconstructions demonstrate that variable amounts of mixing within the free troposphere and about 2% vapor influx mixing per hour from the boundary layer can explain the observed q and δD values. A last saturation model provides a reasonable description of humidity and water vapor isotope ratios of the subtropical free troposphere and results are sensitive to the treatment of mixing of air parcels last saturated in distinctly different regions of the atmosphere.