Acid and base functionalities are believed to be among the dominant functionalities participating in crude oil/brine/rock (COBR) interactions. To improve on our understanding of interactions that might be taking place in COBR systems, this paper focuses on the effects of 4-heptylbenzoic acid (4-HBA) on surface and interfacial tension (IFT) in an oil/water system. The interactions taking place in the 4-HBA/decane/water system were investigated by measuring the partitioning of the acid between the two phases and the changes in IFT as a function of acid concentration, pH and ionic strength. The IFT vs pH measurements were analyzed by the ionizable surface group (ISG) model, combined with Gouy-Chapman theory, to obtain an estimate of the interfacial dissociation constant of the acid. The results presented here show that the model is able to predict IFT as function of pH by considering charging of the interface as the governing mechanism. The influence of Na+ and Ca2+ ion in the aqueous phase on the lowering of IFT by such an interfacial charging mechanism is also demonstrated. The ISG model is thus a useful tool to evaluate changes in the charge at the oil/water interface with pH and salinity, which is of great use in predicting possible interactions in COBR systems. Copyright 1999 Academic Press.