Formation of stable water-in-oil emulsions in light and heavy crude oils was shown to involve the buildup of interfacial layer characterized by increased contents of the main emulsifiers, asphaltenes and resins. An increase in the proportion of asphaltenes in the interfacial layer, nearly 4-fold for the light oil and 1.3-fold for the heavy oil samples, was accompanied by changes in their elemental and structural-group compositions. The proportions of resins in the interfacial layers exceeded by 25–86% those in the original oils. IR-spectroscopic examination revealed increases in the concentration of carboxy and methylene groups in the asphaltenes and resins from the interfacial layer of the light oil and in the concentration of sulfoxide and carboxy groups from the interfacial layer of the heavy sulfurous oil. The dependence of the surface activity of resins and asphaltenes on pH of the aqueous phase was elucidated. Asphaltenes and resins exhibited decreases in interfacial tension (2.3–3.2-fold maximum) in the pH range 10–12 compared to pH range 4–6, which was due to neutralization of their constituent carboxy groups. Reduction of the interfacial tension in the acidic region was attributed to the presence of sulfoxide groups, possessing the properties of weak bases, in the emulsifiers.