Interfacial tension and rheology of water/acidic oil (w/o) interfaces at neutral and alkaline pH conditions have been investigated by means of an oscillating drop tensiometer. The acidic oil phases used for this study were a West-African crude oil and a heavy distilled fraction of this oil, both diluted in cyclohexane to decrease oil viscosity and allow interfacial rheology measurements. In contact with water of initial of 6.5, both oils formed a 2D-gel near its gelation point at the w/o interface. For initial pH beyond 8, ionized molecules like naphthenates formed and adsorbed massively to the w/o interface, modifying the interfacial tension and rheology. We found that naphthenates did not impede but rather delayed the formation of the 2D-gel, and their presence could significantly reduce the gel strength, decreasing the stability of the corresponding w/o emulsions. Because the gel formation was not found to be an instantaneous process, it more surely contributes to the strengthening of emulsions with aging rather to their initial stability.