Abstract-The effect of binary systems composed of 18-phenyloctadecanoic acid (PODA) and each of the following acids: heptanoic (HPA), stearic (SA), and oleic (OA) on the wettability alteration of calcite, IFT and partitioning coefficient is investigated. For SA and OA in single systems in nC10/water, the interfacial pKa is estimated to be at about pH7. While in single system of PODA in n-C10/water, the interfacial pKa is shown to be at a pH > 8. For binary systems of PODA/SA and PODA/OA, the interfacial pKa values for both systems are at a pH > 8, indicating the influence of PODA on the binary system. This may be explained based on hydrophobic intermolecular interaction. However, for PODA/HPA binary system, PODA showed no effect on the pKa. Since HPA is more soluble in the water phase, at high pH a possible repulsion of the ionized molecules occurs. This perhaps reduces/inhibits a complete dissociation of PODA, hence interfacial pKa was not observed.