Abstract Oil can be recovered from fractured, initially oil-wet carbonate reservoirs by wettability alteration with dilute surfactant and electrolyte solutions. The goal of this work is to study the effect of salinity, surfactant concentration, electrolyte concentration, and temperature on the wettability alteration and identify underlying mechanisms. Contact angles, phase behavior, and interfacial tensions were measured with two oils (one model oil and one field oil). There exists an optimal surfactant concentration for varying salinity and an optimal salinity for varying surfactant concentration at which the wettability alteration is the maximum for anionic surfactants. As the reservoir salinity increases, the extent of maximum wettability alteration decreases for a surfactant, but the surfactant concentration needed for the maximum wettability alteration decreases. IFT and contact angle were found to have the same optimal salinity for a given concentration of anionic surfactants. As the ethoxylation increases in anionic surfactants, the extent of wettability alteration increases. Wettability of carbonates can be altered by divalent ions at high temperature (90°C and above). Sulfate and calcium ions play a more important role in altering wettability than magnesium ions. Wettability alteration increases the oil recovery rate from fractured carbonates.