Improved oil recovery from oil-wet, low-permeability, and fractured carbonate reservoirs is a great challenge. Water injection based on spontaneous imbibition from the fractures into the matrix blocks, in combination with a wettability alteration process, seems to be an actual method. The present paper adds more important information about the mechanism for wettability alteration of oil-wet carbonate rock, using cationic surfactants of the alkyltrimethylammonium type, in regard to brine salinity and composition. The oil recovery at low temperature (40 degreesC) is delayed as the salinity increases, because of a decrease in the critical micelle concentration of the surfactant, but the salinity effect seems to vanish at higher temperatures (70 degreesC). In chalk, improved oil recovery has been observed when sulfate is added to the imbibing fluid, especially at low temperatures (40 degreesC). At higher temperatures (70 degreesC), the effects of sulfate also vanish in this case. The catalytic effect of sulfate is of minor importance when dolomite cores are used. A model for the catalytic effect of sulfate is suggested and confirmed by advancing contact-angle measurements on calcite, dolomite, and magnesite crystals.