This study attempts to quantify the interface between flow domains in macroscopic dual-porosity flow and transport models. For a structured soil, this interface could be related to the surfaces of soil aggregates and biopores that may serve as preferential flow paths. Mass exchange with the soil matrix domain across this interface requires including the combined effect of various structural shapes and sizes, which has not yet been properly defined. The objective was to test an approach for independent soil structure quantification by characterizing domain interfaces based on surface area/volume ratios using two soil samples with contrasting structures. X-ray computed tomography data from undisturbed soil cores from a medical scanner were analyzed. For a range of density threshold values, surface areas were identified on a voxel basis. The resulting functions were found to be characteristic for the subsoil sample with cylindrical pinhole pores as well as for the aggregated topsoil sample. The results suggest that the interdomain interface area could be used for deriving mass exchange coefficients and a macroscopic characteristic related to visual inspection of the soil structure. The approach, representing an upscaling of individual structural features to the macroscopic scale, may improve the applicability of two-domain models by finding a way to obtain relatively simple estimates of the geometry coefficient from qualitative soil structure information given in soil protocols. Although this analysis did not allow determination of a well-defined interface between porous domains, it enabled narrowing of the range of possible values in a physically plausible region. Still more effort is required to identify specific types of structural surfaces among other features.