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In this work, the constitutive relationship between capillary pressure (P_c), saturation (S_w), 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 P_c − S_w curve to the total IFA, a_n, and the capillary‐associated IFA, a_nw. Surfaces were fit to the experimental and modeled P_c − S_w − a_n and P_c − S_w − a_nw data in order to characterize the relationship in three dimensions (3D). For the experimental system, it was shown that the P_c − S_w − a_n relationship does not exhibit hysteresis. The model is found to provide a reasonable approximation of the magnitude of the 3D surfaces for a_n and a_nw, with a mean absolute percent error of 26% and 15%, respectively. The relatively …