Natural surfactants (NS) have been isolated from extra heavy crude oils and from their asphaltene and maltene fractions. The elemental analyses and molecular weight (M,) properties and the molecular weight distribution (MWD), as well as the interfacial tension (y) behaviour (toluene solutions) with pH have been measured. The NS of crude oils (NSC) and those from asphaltenes (NSA) presented properties similar to those of asphaltenes (high M,, low H/C ratio, heteroatom content and Y-pH behaviour) whereas the NS isolated from maltenes (NSM) resembled neither asphaltenes nor resins. The Y-pH behaviour of the crude oil systems were not reproduced by their fractions (asphaltenes, maltenes, NSC, NSA or NSM ). A chemical equilibrium, based on ion pairs formed between carboxylic acids and amines, is proposed to account for the observed Y-pH changes.