An experimental method has been developed to determine-the wettability, i.e., the contact angle, of a reservoir brine-reservoir rock system with dissolution of M at high pressures and elevated temperatures by using the axisymmetric drop shape analysis (ADSA) technique for the sessile drop case. Prior to the experiment, a rock slide is horizontally placed in a specially designed rock slide holder in a see-through windowed high-pressure cell, which is subsequently filled with CO2 at a prespecified pressure and a constant temperature. Then, a reservoir brine sample is introduced by using a syringe delivery system to form a sessile brine drop on the rock slide inside the pressure cell. The sequential digital images of the dynamic sessile brine drop are acquired and analyzed by applying computer-aided image acquisition and processing techniques to measure the dynamic contact angles at different times. It is found that the dynamic contact angle between the reservoir brine and the reservoir rock remains almost constant at a given pressure and a constant temperature, though CO2 is gradually dissolved into the sessile brine drop which is eventually saturated with CO2. It is also found that the equilibrium contact angle increases as the pressure increases, whereas it decreases as the temperature increases. Such wettability alteration may significantly affect the storage capacity when CO2 is injected into a saline aquifer or a depleted oil reservoir at high pressures.