Wettability alteration appears to be the main mechanism of low salinity water flooding in carbonate reservoirs. However, what factor(s) controls the wettability alteration is not clearly defined. We hypothesized that zeta potential at interfaces of oil/brine and brine/rock controls the wettability alteration in carbonate reservoirs. To test our hypothesis, we removed SO42- ions from the aqueous ionic solutions because SO42- ions likely adsorb at the pore surface, triggering desorption of carboxylic materials. We examined the zeta potential of interfaces of crude oil/brines and brines/rock. We also measured the contact angle and conducted two core-flooding experiments. Moreover, we performed a geochemical study to examine the potential of calcite dissolution by low salinity water using PHREEQC software. Our work demonstrates that contact angle strongly correlates with polarity of the zeta potential of interfaces of oil/brine and brine/rock, showing that the same polarity of zeta potential for oil/brine and brine/rock triggered a strongly water-wet carbonate surface in the presence of formation brine. The opposite polarity of zeta potential of oil/brine and brine/rock, on the other hand, yielded an intermediate or slightly oil-wet surface in the presence of 10 times diluted formation brine. However, incremental recovery of 5.8-18.1% was observed under tertiary mode, implying that low salinity water likely shifts the reservoir wettability from strongly water-wet to intermediate-wet. In addition, geochemical modeling revealed a negligible amount of calcite was dissolved in the presence of low salinity water, confirming that calcite dissolution is not a contributing factor to the low salinity effect. Moreover, we demonstrate that the contact angle on the carbonate substrate decreases linearly with increasing Z parameter, Z(p) = (vertical bar Zeta(oil/brine) + Zeta(brine/rock|)|). We therefore argue that manipulating the Z parameter (polarity of zeta potential of brine/rock and oil/brine) is of vital importance to shift the reservoir wettability and improving oil recovery. We suggest that the Z parameter may be used as an interpolant to model the low salinity effect, rather than using salinity level or individual ions in the solution.