Effect of Mg2+ and SO42- on wettability alteration of modified calcite surface to oil-wet by stearic acid (SA) is addressed both in macroscopic and nanoscale using contact angle and atomic force microscopy technique (AFM), respectively. No apparent difference is shown by AFM images, compared to a clear trend that is obtained form contact angle measurements, where Mg2+ ions have shown to alter the modified calcite surface to more water-wet than that in presence of SO42- ions. The adhesion forces, due to the presence of SA, are shown to be less pronounced in presence of M2+ ion than that in case of SO42-. This confirms the macroscale measurements of contact angle by nanoscale level. The phenomenon of the alteration to more water-wet calcite surface is related to the distribution coefficient of SA in n-decane/water system, which decreased in presence of Mg2+ and SO42-ions, indicating less adsorption of SA on calcite surface. (c) 2007 Elsevier Inc. All rights reserved.