Nearly half of all known oil reserves are held in oil-wet, fractured carbonate reservoirs. Wettability alteration towards more water-wet state reduces the capillary barrier, hence improving oil recovery efficiency in such reservoirs. The aim of this work is to investigate the effect of sodium carbonate and sodium metaborate as alkaline solutions in the absence and presence of anionic/nonionic surfactant mixture on the wettability of oil-wet carbonate surfaces. Calcite surfaces were aged in model oil and crude oil and treated with different alkaline solutions. Contact angle and zeta potential measurements were used as a mean to infer wetting conditions before and after treatment. In addition, phase behavior, interfacial tension measurements and core flooding experiments were conducted to examine the ability of these solutions to enhance oil recovery from oil-wet limestone cores.Contact angle measurements for calcite surfaces aged in model oil and crude oil showed oil-wet conditions. Treatment of these surfaces with alkaline solutions showed negligible change in the wettability of surfaces aged in model oil. Sodium carbonate/anionic-nonionic surfactant solution altered the wettability of these surfaces to intermediate to water-wet state. On the other hand, contact angle measurements revealed that both alkalis were effective in changing the wettability of oil-wet calcite surface aged in crude oil. Sodium carbonate in the presence of anionic surfactant altered the wettability towards more water-wet state, whereas for sodium metaborate an opposite effect was observed and an intermediate wetting condition was established.Zeta potential measurements indicated a possible ion exchange between metaborate (BO2) and/or (OH) ions and the adsorbed carboxylate groups on the surface as well as interaction between these ions and the calcium that is attached to thecarboxylate that could lead to the release of carboxylate from these surfaces. For anionic surfactant, surfactant molecules could partially adsorb on the carboxylate groups through hydrophobic-hydrophobic interactions between the tail of the surfactant and the adsorbed carboxylate from the crude oil.Additional oil recovery of around 25 % of original oil in place was achieved using alkaline and alkaline-surfactant mixture (around 11-22% from alkaline alone and 810% from alkaline-surfactant mixture). Wettability alteration toward water-wet state and IFT reduction are contributing factors for additional oil recovery from oil-wet limestone cores. Aging the core samples after injection of the different brine and chemical fronts indicated the importance of time for the crude oil, rock and brine interactions and the consequent additional oil recovery.