Although various efficient enhanced oil recovery (EOR) techniques have been proposed, the use of chemicals such as alkali, surfactants, and polymer in the field of EOR makes chemical EOR a promising method. The residual oil left behind after the secondary recovery process can be successfully displaced by decreasing the interfacial tension (IFT) between the liquid-liquid systems. Moreover, understanding the IFT between the liquid-liquid systems are essential in formulating and controlling the multiphase and multicomponent processes. As crude oil is a complex mixture of organic, inorganic compounds and heteroatoms, understanding the IFT of the pure hydrocarbon-water system is vital to develop robust models. In this study, the IFT between pure hydrocarbon-water systems in the presence of alkali has been explored. The IFT measurements were performed using a dynamic contact angle tensiometer using a Wilhelmy plate. Different hydrocarbon liquids such as hexane, heptane, decane and aromatics such as benzene and toluene were used. The monovalent NaOH and divalent alkali Ca(OH)2 in the concentration range of 0–37.5 mM are used to understand the effect of alkali on the IFT of the oil-water system. This paper also elaborates on the effect of temperature at 298.15 K, 323.15 K and 348.15 K on the IFT of hydrocarbon-water systems. The experimental results show that the increase in the concentration of alkali continuously decreased the IFT of hydrocarbon-water systems. Moreover, it has been found that the type of alkali also has a reasonable impact on the IFT of the hydrocarbon-water system. The divalent alkali [Ca(OH)2] is found to be more efficient in reducing the IFT of both alkane-water and aromatic-water systems than the moderately performed monovalent alkali (NaOH). The possible mechanism for the continuous reduction in IFT has been proposed using surface adsorption of hydroxide ions at the interface of the oil-water system. © 2020 Elsevier B.V.