The concerns for water availability, affordability and environmental consideration have motivated more research on the development of smart water injection for enhanced oil recovery process. Although wettability alteration has been considered as the dominant mechanism, there is an ample space in this area needs to be explored more. Therefore, a systematic series of experiments is designed and performed to examine the effect of salinity and ion type on the wettability of the carbonate rock surface to find the active mechanisms. For this purpose, the concentrations of different salts including NaCl, KCl, CaCl2 and MgCl2 are examined during 10 days of soaking for salts concentrations range of 0-45,000 ppm. Based on the measurements of contact angle (CA), it is concluded that the rnonovalent cation bonded to the chloride anion showed better performance on the wettability alteration of oil wet carbonate rock surface compared to the divalent cations bonded to the chloride anion. In addition, comparing the results of CA measurements of NaCl and KCl solutions as a function of concentration showed completely different wettability behaviors. The presence of KCl shifted the wettability of the carbonate rock surfaces from strongly oil wet to strongly water wet state for all of studied concentrations while as the concentration of NaCl was decreased, the more wettability alteration was observed. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.