Knowledge of interfacial properties, including both fluid-fluid interfacial tension and mineral wettability is essential for accurate simulation of carbon dioxide storage in geological formations. In this context, carbonate reservoirs, especially saline aquifers, are of great interest due to their vast storage capacities; therefore, it is imperative to attain a thorough understanding of their wettability under the high-pressure, high-temperature (HPHT) conditions of CO 2 storage. To this purpose, contact angles have been measured for the system CO 2 + NaHCO 3 (aq) + calcite under HPHT conditions. Calcite is representative of limestone minerals and the brine chemistry and molality (1 mol·kg −1 ) have been chosen to inhibit dissolution reactions. Both static (sessile drop) and dynamic (tilting plate) contact angle measurements were carried out under reaction-free conditions at temperatures from (298 to 373) K and at pressures up to 30 MPa. The influences of surface roughness and cleanliness have also been addressed in this study. We found that calcite is mainly brine-wet, but it can turn intermediate-wet or even weakly CO 2 -wet at intermediate pressures (around 10 MPa) and low temperature conditions (around 300 K). The results presented in this work may prove useful for characterizing the wettability of a wide variety of calcite (limestone) surfaces that one might expect to encounter in natural reservoirs.