Whole asphaltenes (WA) were fractionated by the E-SARA method according to their adsorption characteristics at oil-water interfaces from either toluene or heptol solutions. Heptol, a mixture of n-heptane and toluene at a 1:1 volume ratio, is a less aromatic solvent than toluene. The effect of solvent aromaticity on the composition of resulting asphaltene subfractions at oil-water interfaces was studied to determine the key functional groups that are critical to the asphaltene-induced stabilization of water-in-oil (W/O) petroleum emulsions. The interfacially active asphaltenes (IAA) were extracted as materials irreversibly adsorbed onto emulsified water droplets, while the asphaltenes remaining in the oil phase were considered as remaining asphaltenes (RA). Although toluene-extracted interfacially active asphaltenes (T-IAA) accounted for only 1.1 +/- 0.3 wt % of WA, this subfraction of asphaltenes exhibited a greater interfacial activity and formed more rigid films at the oil-water interface than IAA extracted using heptol, known as HT-IAA which accounted for 4.2 +/- 0.3 wt % of WA. The increased potential of T-IAA to stabilize W/O emulsions was attributed to their higher content of oxygen, resulting in a higher content of sulfoxide groups, as verified by elemental analysis, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Although the toluene-extracted remaining asphaltenes (T-RA) and heptol-extracted remaining asphaltenes (HT-RA) were shown to contain similar H/C ratios and nitrogen contents to those of T-IAA and HT-IAA, the two RA subfractions contained a much less amount of sulfur and oxygen, leading to a much reduced interfacial activity as compared with that of IAA subfractions. In spite of the small proportions in asphaltenes, oxygen-containing functional groups, in particular sulfoxides, were believed to contribute significantly to the increased stability of asphaltene-stabilized W/O petroleum emulsions.