Modifying the composition of injection brine can cause wettability alteration and lead to enhanced oil recovery (EOR) from carbonate reservoirs. The two main approaches are: (1) dilution of injected brine and (2) selective modification of the concentration of potential determining ions (PDIs) such as Ca2+, Mg2+ and SO42-. Each of these approaches will modify the surface charge of carbonate mineral expressed in terms of the zeta potential. Here we reports measurements of the zeta potential of intact carbonate samples obtained using the streaming potential method. This method allows us to directly link zeta potential and oil recovery in the same coreflooding experiments. We begin by using single-phase (brine only) experiments to investigate the effect on zeta potential of dilution and PDI modification using seawater as a reference injection brine composition. We then use multi-phase experiments (brine displacing oil) and carbonate samples at different initial wetting states to determine the correlation between injection brine composition, zeta potential, EOR and initial wetting state, using the same brine compositions as in the single-phase experiments. We find that typical formation brine yields a positive zeta potential, whilst seawater yields a negative zeta potential. Consequently, injection of seawater leads to a change in the polarity of the zeta potential and therefore affects fluid-rock interactions. This in turn affects oil recovery. Dilution of injected seawater and an increase in SO42- concentration both yield a more negative zeta potential compared to the seawater reference case. However, the change in zeta potential observed when adding SO42- is less than for bulk dilution. The magnitude of the zeta potential increases in the presence of an oil phase after aging and similar increase was observed on diluting the injection brine. In the experiments reported here, incremental oil production was not observed. We suggest that the increase in the zeta potential, and consequent increase in the electrostatic repulsion between the similarly charged mineral-brine and oil-brine interfaces, explains why most of the oil is produced during first brine injection. Other samples, which were not aged, yield no change in zeta potential compared to water-wet cases within experimental error.