The relationship between air-water interfacial area and water saturation was investigated for two natural porous media.  Air-water interfacial areas as a function of water saturation were measured using synchrotron X-ray microtomography, which can be used to determine both capillary-associated and total areas.  Total air-water interfacial areas increased continuously with decreasing water saturation.  Conversely, capillary-associated interfacial areas first increased and then decreased with water saturation.  Interfacial areas obtained from the microtomography method were compared to values obtained with gas-phase and aqueous-phase partitioning tracer tests.