Abstract — Understanding the transport of three fluid phases through porous media has important applications in subsurface contaminant remediation, oil and gas recovery, and geological CO₂ sequestration. Existing transport models may be improved by including physical phenomena that govern fluid flow at the pore scale. In particular, thermodynamic arguments suggest that hysteresis in the capillary pressure-saturation (P[subscript c]-S) relationship may be resolved by including an additional parameter, fluid-fluid interfacial area per volume (a[subscript nw]). Synchrotron-based Computed X-ray Microtomography (CMT) is a method that allows observation of fluid interfaces. Flow experiments were conducted using CMT to investigate uniqueness of the P[subscript c]-S[subscript w]-a[subscript nw] relationship in a porous media system containing three immiscible fluid phases. Drainage and imbibition surfaces were fit to P[subscript c]-S[subscript w]-a[subscript nw] data collected over a limited range of water saturations. The root-mean-square error (RMSE) between the drainage and imbibition surfaces was negligible, indicating that the P[subscript c]-S[subscript w]-a[subscript nw] relationship is unique. These results are a first step in validating the P[subscript c]-S[subscript w]-a[subscript nw] relationship for three-phase porous media systems. In addition, spreading intermediate-phase layers were observed to bring oil and solid into contact, which in the presence of X-rays changed the solid wettability within a relatively short time period. These observations confirm a proposed theoretical scenario that three-phase systems are more susceptible to wettability changes than to two-phase systems due to intermediate-phase spreading behavior.