In this study. dual-drop dual-crystal (DDDC) contact-angle measurements have been made using dolomite rock and fluid samples from the Yates reservoir (West Texas) and in the presence of an anionic (ethoxy sulfate) surfactant. The experiments have been conducted at Yates reservoir conditions (4.8 MPa and 27.8 degrees C) and using live synthetic oil to provide realistic measurements of in situ reservoir wettability. Stocktank crude oil has also been used at reservoir conditions to study the oil compositional effects on wettability. An advancing contact angle of 152 degrees measured for Yates dolomite rock, stocktank oil and synthetic reservoir brine showed a strong oil-wet nature. However. experiments with Yates live synthetic oil resulted in an advancing contact angle of 55 degrees. indicating a weakly water-wet behavior. In the rock-fluids system consisting of Yates stocktank oil. the surfactant altered the wettability to less oil-wet by decreasing the advancing contact angle to 135 degrees. For rock-fluids system with Yates live synthetic oil, the surfactant altered the wettability from weakly water-wet to strongly oil-wet by increasing the advancing contact angle from 55 degrees to 165 degrees. The oil-wet behavior observed with Yates live synthetic oil due to the surfactant indicates a significant wettability altering capability of the surfactant.