The wettability of oil reservoir rock affects the efficiency of the oil recovery process by reducing the capillary force. Methyldodecylbis [2-(dimethyldodecylammonio) ethyl] ammonium tribromide is a trimeric cationic surfactant that contains three dodecyl chains and three quaternary ammonium head groups connected by divinyl groups. The surfactant was synthesized, purified and used as a new wetting alteration agent. This paper focuses on the ability of this trimeric cationic surfactant to alter the wettability of water-wet and oil-wet mica mineral surfaces. The contact angle data of the solid-liquid interface in oil/water/solid three-phase system show that the trimeric cationic surfactant, when compared with single-and double-chain cationic surfactant, is a more effective wetting agent for water-wet and oil-wet mica surfaces at lower concentration. Measurements by atomic force microscopy (AFM) show that the surfactant molecules have formed a monolayer to reverse the wetting properties. On the water-wet surface, the surface is suffused with negative charge, which could attract the cationic head of surfactant, and leave the hydrophobic tails exposed. In contrast, on the oil-wet surface, the hydrophobic tails were attracted by hydrophobic interactions to the oil film between the surfactant and the crude oil. The hydrophilic heads were left outside to form a hydrophilic layer, which could explain the wettable to hydrophilic trend. Alteration to the degree of wettability is mainly dependent on the adsorption areas of the surfactant. The data show that the ability of the trimeric cationic surfactant affect the wettability is independent of surface tension. (C) 2012 Elsevier B.V. All rights reserved.