The effect of brine ionic composition on oil recovery was studied for a limestone reservoir rock at a high temperature. Contact angle, imbibition, core flood and ion analysis were used to find the brines that improve oil recovery and the associated mechanisms. Contact angle experiments showed that modified seawater containing Mg2+ and SO42- and diluted seawater change aged oil-wet calcite plates to more water-wet conditions. Seawater with Ca2+, but without Mg2+ or SO42- was unsuccessful in changing calcite wettability. Modified seawater containing Mg2+ and SO42-, and diluted seawater spontaneously imbibe into the originally oil-wet limestone cores. Modified seawater containing extra SO42- and diluted seawater improve oil recovery from 40% OOIP (for formation brine waterflood) to about 80% OOIP in both secondary and tertiary modes. The residual oil saturation to modified brine injection is approximately 20%. Multi ion exchange and mineral dissolution are responsible for desorption of organic acid groups which lead to more water-wet conditions. Further research is needed for scale-up of these mechanisms from cores to reservoirs.