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Stable water-in-diluted-bitumen emulsions are detrimental to the commercial production of Athabasca bitumen. Isolation and characterization of interfacial materials (IM) from these emulsions are vital in order to reveal the emulsion stabilization mechanism, which is not fully understood to date. Recently, a heavy water method to isolate IM has been developed for model systems containing less than 10% bitumen. In this study, the heavy water method was improved to isolate IM from water-in-naphtha-diluted-bitumen emulsions at a bitumen concentration typically used in the commercial operation (59 wt %). Modification of the method includes an additional naphtha cleaning process for the wet IM cake. The isolated IMs were characterized by a variety of techniques such as elemental analysis, density measurements, thermal gravimetric analysis, infrared spectroscopy, Langmuir trough, and atomic force microscopy. The results show that the IMs isolated from emulsions containing 5% and 59% bitumen, referred to IM5 and IM59 respectively, are quite different. IM59 on surface of water droplets is likely in a monolayer structure, while IM5 in a multilayer structure. Asphaltene-like species could be dominant in IM5. IM59 features a higher H/C ratio and contains more volatiles, more chloroform insoluble components, and clays as compared to IM5. Carboxylates, probably in the form of sodium naphthenates, were detected in IM59 only, which may play a role in monolayer formation in the emulsion from which IM59 was obtained. Combination of monolayer structure and clays is believed to cause the high stability of emulsions at the bitumen concentration of 59%.