Recent studies have shown how water films at near-zero matric potentials on fracture surfaces could permit fast flow in unsaturated fractures. The present work is aimed at delineating the ranges of conditions necessary to permit film flow. The approximate matric potential needed to permit thickening of water films on fracture surfaces was estimated through correlating air-entry matric potentials and permeabilities for a wide range of porous media. An upper matric potential limit, associated with saturating the fracture, was related to the fracture aperture. These two limits, one dependent on matrix permeability and the other on fracture aperture, delineate the matric potential range within which thick water films develop on fracture surfaces. Rocks with matrix permeabilities less than ~10-14 m2, and fractures with apertures greater than ~30 &mgr;m permit some matric potential range over which thick water films can form. Highly transmissive films can develop on fracture surfaces only under near-zero matric and pressure potentials. ¿ 2001 American Geophysical Union