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Abstract It is widely believed that asphaltenes and other high molecular weight, polar components of crude oils are responsible for altering the wetting of reservoir rocks, but the mechanisms by which wetting is affected are still uncertain. The concentration of asphaltenes in an oil is not necessarily a good predictor of rock/oil interactions. The composition of the remainder of the oil phase is equally important, particularly with regard to its properties as a solvent for some of its largest constituents. Solvent quality of the crude oil mixture is a function of composition, temperature, and pressure. Changing solvent quality by addition of an excess volume of low molecular weight paraffins is the basis for definition of the asphaltenes, the material that precipitates from the diluted mixture. The extent to which wetting is altered in oil/brine/rock systems by adsorption of asphaltic material from the crude oil is affected by changes in oil solvent properties, even in mixtures in which the asphaltenes are stable with respect to precipitation. A simple approach, using measurements of mixture refractive index, is applied to quantify oil solvent quality for mixtures of crude oil or asphaltenes with hydrocarbon additives. Wettability alteration with systematic changes in oil composition is evaluated by contact angle measurements. Crude oils generally induce greater alteration as they become poorer asphaltene solvents.