In oil sand industry, formation of a stable salty water-in-diluted bitumen emulsion poses a serious corrosion problem at the bitumen upgrading plant. Previous studies indicated the presence of a "critical concentration" of bitumen in the emulsion, above which the oil-water interface is flexible and the water-in-diluted bitumen emulsion is stable, but below which the interface becomes rigid and the emulsion is unstable. The cause of this critical concentration was investigated through isolation and characterization of the interfacial material from the emulsions both below and above the critical concentration. The analytical data showed that, in heptane/toluene(1:1)-diluted bitumen emulsions, the flexible interfacial film is composed of a mixture of asphaltene and carboxylic salts with a combined H/C ratio of 1.32 while the rigid interfacial film is composed of asphaltene alone with a H/C ratio of 1.13. The carboxylic salts are water insoluble and are likely sodium naphthenates containing >20 carbons. The presence of sodium naphthenates might hinder the conformational change of asphaltene at the interface, necessary for rigid film formation.