The properties of asphaltenes derived from Venezuelan vacuum residues have been investigated using several techniques. X-ray diffraction and Differential Scanning Calorimetry (DSC) and infrared spectrometry results indicate that asphaltenes are complex mixtures that form nanostructures with some (low) degree of ordering, probably consistent with a sheet-like arrangement of aromatic nuclei. Nanostructures seem to be also present in solutions of the asphaltenes in toluene, and an increase in their concentration can induce phase separation by flocculation. Asphaltene aggregates show an isolelectric point at around pH=4 and their charge is highly negative at high pH. This is attributed to the presence of ionisable acidic groups that also are responsible of low water/toluene interfacial tensions at basic conditions. A decrease in interfacial tension is also induced by increasing asphaltene concentration or the addition of electrolyte. The results are correlated with the stability of water-in-oil emulsions, which seems to be determined by interfacial adsorption.