Various experimental methods including atomic force microscopy, scanning electron microscopy, zeta potential measurement, and contact angle measurement were used to analyze the mechanisms of wettability alteration of an oil-wet sandstone surface by cationic/nonionic surfactant mixtures in this work. Due to the synergies between cationic surfactants and nonionic surfactants, head groups of CTAB and TX-100 interact with each other, making the cmc of the cationic/nonionic surfactant mixtures lower compared to the single surfactant CTAB or TX-100. Ion pairs are produced by the carboxylic substances and the aggregates formed by CTAB and TX-100, which are irreversibly desorbed from the quartz surface and are solubilized into the mixed micelles formed by the CTAB/TX-100 mixture. The CTAB molecules are preadsorbed on the oil-wet sandstone surface by electrostatic attraction, acting as anchor particles, and the aggregates are formed by TX-100 and CTAB through hydrophobic interaction, thereby increasing the adsorption amount of CTAB on the oil-wet sandstone surface. The ability to form ion pairs for CTAB and the carboxylic substances in the presence of TX-100 and the solubilization ability of the mixed micelles are all enhanced, making the desorption capacity of ion pairs stronger. Thus, the CTAB/TX-100 mixture is more effective than the single surfactant CTAB in altering wettability of the oil-wet sandstone surface toward a more water-wet condition.