It was previously documented that seawater was able to change the wettability of oil-saturated chalk toward more water-wet conditions and that enhanced spontaneous imbibition of water was observed. The efficiency of the imbibition process was improved by increasing the sulfate concentration in seawater. Both calcium and sulfate present in seawater are potential determining ions toward the chalk surface; it is therefore expected that both of the ions are involved in the wettability modifying process, and the symbiotic effects between the ions are studied in this paper with the aim of improving oil recovery from moderate water-wet chalk. Outcrop chalk samples were aged at 90 °C in an acidic crude oil for at least 4 weeks. The concentration of Ca2+ was varied both in the imbibing seawater and in the initial brine. Chromatographic wettability tests showed that the initial wetting condition of the chalk was not significantly affected by changing the concentration of Ca2+ in the initial brine. Increased oil recovery by spontaneous imbibition was observed with increasing concentration of Ca2+ in the imbibing seawater or increasing concentration of Ca2+ in the initial brine. The imbibition increased as the temperature increased, but care must be taken to avoid precipitation of CaSO4(s) at high temperatures. The spontaneous imbibition tests confirmed that SO4 2- and Ca2+ played an important role in the wettability modifying process. A chemical mechanism for the wettability alteration was suggested to involve the coadsorption of SO4 2- and Ca2+ onto the chalk surface, which resulted in desorption of carboxylic material.