Altering the wettability of rock surface to a favorable wetting condition is one of the main mechanisms of enhanced oil recovery methods applied in carbonates. In order to obtain desired wetting state, it is essential to have a clear understanding of the original wetting properties of the rock surface. This study used different measurements to investigate the individual effects of an organic acid and a base at different concentrations as well as their combinations on wettability alteration of calcite surface. Wettability of the calcite was examined before and after treating the surfaces with different model oil solutions using contact angle and zeta potential tests. Besides, Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) were used to recognize the adsorbed functional groups and the amount of adsorbed organic compounds on the calcite surface, respectively. Moreover, interfacial tension (IFT) measurements were used to evaluate the interfacial activity of the model oil systems. Stearic acid represented the crude oil acidic component while N,Ndimethyldodecylamine (N,N-DMDA) represented the basic component of crude oil. Different model oil systems were used for acid-base combinations in which the acid concentration was kept constant for each system while the base concentration was adjusted to prepare base to acid ratios of 0.25, 0.5, 1, 2 and 4. The results revealed that by increasing the acid number, wettability of calcite surface shifted toward more oilwetness and adsorption of stearic acid on calcite surface increased. Stearic acid with low concentration (AN=0.25) was enough to alter the wettability of calcite surface from water-wet to oil-wet. TGA results showed vertical adsorption of stearic acid molecules on the calcite surface. Treating calcite surfaces with the base revealed that low concentration of N,N-DMDA (BN=0.25) was able to shift the calcite wettability from water-wet to slightly oil-wet. It was observed that as BN increases, wettability alteration toward oil-wetness is slightly improved. TGA results showed low adsorption of the base with high surface area occupied by the base molecule which indicate that the base is oriented horizontally on the calcite surface. Possible mechanisms of wettability alteration are proposed based on possible interactions between the base molecules/cations and calcite surface. For acid-base mixtures, at constant AN, the wettability alteration toward oil-wetness increases in the presence of the base until BN/AN ratio is equal to 1, then oil-wetness decreases. IFT measurements indicated lower IFT of acid-base mixtures compared to those of individual acidic and basic model oil solutions which could be due to synergistic effect between acid and base by forming acid-base complexes. The presence of the base hindered the acid adsorption the surface. However, the basic component could contribute to the wettability alteration by forming acid-base complexes which can adsorb on the calcite surface and impact its wetting preferences. In addition, the base molecules may adsorb on the calcite surface at high BN/AN. Hence, ratio of BN to AN would determine the contribution of acid molecules, base molecules and acid-base complexes in wettability alteration of calcite surfaces. Mechanisms of wettability alteration by acid-base combinations at different BN/AN ratios are presented.