The impact of brine composition on rock wettability and oil recovery in carbonates has been an area of research in recent years. Many studies have reported contradictory results concerning the impact of water injection salinity and composition on oil recovery. The zeta potential, which is a measure of the electrical charge at the mineral surface, is highly variable in carbonates, depending on the ionic composition of the pore water. The zeta potential controls the magnitude and polarity of the electrostatic interactions between the mineral surface and polar species in the brine and oil; it also controls the magnitude and polarity of the streaming potential, an electrical potential which arises in response to pressure gradients across saturated rocks. Here we report the use of streaming potential measurements to characterize wettability and optimize injection brine composition during controlled salinity waterflooding (CSW) in carbonates. Crude oils, natural carbonate core samples and synthetic brines (equivalent to formation, seawater and modified seawater compositions) are used to evaluate wettability and CSW effect. We use the streaming potential measurements to determine the zeta potential, and correlate changes in zeta potential with changes in wettability and improved oil recovery. To predict the optimum brine composition for CSW requires knowledge of the zeta potential and how this responds to changes in brine composition. Such knowledge can be obtained using the streaming potential method reported here, which is much cheaper and quicker than conducting numerous multiphase coreflooding experiments and varying the brine composition on an ad-hoc basis.