Water chemistry has a profound effect on the stability of the water film and desorption of organic oil components from the mineral surfaces in a water—based enhanced oil recovery (EOR) process. By knowing the chemical mechanism for the symbiotic interaction between the active ions in seawater; Ca2+, Mg2+, and SO42—, "Smart Water" can be designed and optimized in terms of salinity and ionic composition as an EOR—fluid. In the present study, the efficiency of "Smart Water" on oil recovery in carbonate rock is summarized and discussed in the temperature range of 70—120 °C. By removing non–active salt, NaCl, from the composition of the injected seawater, the oil recovery by spontaneous imbibition was improved by about 5–10% of OOIP compared to seawater. On the other hand, an increase in the concentration of NaCl in the imbibing fluid resulted in reduced oil recovery of 5% of OOIP. A systematic decrease in oil recovery was observed when diluting seawater with distilled water to obtain a low salinity brine, 1000–2000 ppm. The decrease in the oil recovery was attributed to the reduced concentration of the active ions. When seawater depleted in NaCl was spiked by SO42–, the oil recovery increased by about 5 to 18% of OOIP compared to seawater depleted in NaCl. The amount of Ca2+ in the seawater depleted in NaCl had no significant effect on the oil recovery at low temperatures, ≤100 °C, but improvements were observed at higher temperatures. The wetting condition after spontaneous imbibition of seawater, seawater depleted in NaCl, and seawater depleted in NaCl spiked with sulfate was monitored using a chromatographic wettability test, and it showed an increasing order in the water–wet fraction, in line with increased oil recovery. The results were discussed in terms of the previously suggested mechanism and the effect of the ionic double layer at the rock–brine interface.