In this work, the constitutive relationship between capillary pressure (Pc), saturation (Sw), and fluid-fluid interfacial area per volume (IFA) is characterized using computed microtomography for drainage and imbibition experiments consisting of a nonaqueous phase liquid and water. The experimentally measured relationship was compared to a thermodynamic model that relates the area under the Pc − Sw curve to the total IFA, an, and the capillary-associated IFA, anw. Surfaces were fit to the experimental and modeled Pc − Sw − an and Pc − Sw − anw data in order to characterize the relationship in three dimensions (3D). For the experimental system, it was shown that the Pc − Sw − an relationship does not exhibit hysteresis. The model is found to provide a reasonable approximation of the magnitude of the 3D surfaces for an and anw, with a mean absolute percent error of 26% and 15%, respectively. The relatively high mean absolute percent errors are primarily the result of discrepancies observed at the wetting- and nonwetting-phase residual saturation values. Differences in the shapes of the surfaces are noted, particularly in the curvature (arising from the addition of scanning curves and presence of an − Sw hysteresis in the predicted results) and endpoints (particularly the inherent nature of thermodynamic models to predict significant anw associated with residual nonwetting-phase saturation). Overall, the thermodynamic model is shown to be a practical, inexpensive tool for predicting the Pc − Sw − an and Pc − Sw − anw surfaces from Pc − Sw data.