A THIN reservoir of free aqueous or carbonic fluids at mid-crustal levels, perhaps in the form of water sills(1), has been suggested as the source of fault-transmitted metasomatizing fluids(2-4), as the cause of observed deep crustal electrical conductivity and anomalous seismic reflectivity(5), and as a lubricated potential detachment zone for thin-skin tectonics(6). A problem with this hypothesis is that estimated crustal permeabilities are too high to permit long-term retention of such fluids(7,8). Most mechanisms suggested for achieving the required permeability reduction rely on maintaining unusually low porosity(2,6,9,10). Because porosity creation, by tectonic deformation, fluid release or infiltration, is a ubiquitous process, permeability reductions achieved by these mechanisms are hard to sustain for long periods of time. A sealing (permeability reduction) mechanism that can operate in the presence of significant porosity is required. Here I propose that the required mechanism is capillary sealing by immiscible CO2-H2O mixtures derived from rising volatiles.