The success of CO2 storage in deep saline aquifers and depleted oil and gas reservoirs is largely controlled by interfacial phenomena among fluid phases and rock pore spaces. Particularly, the wettability of the rock matrix has a strong effect on capillary pressure, relative permeability, and the distribution of phases within the pore space and thus on the entire displacement mechanism and storage capacity. Precise understanding of wettability behavior is therefore fundamental when injecting CO2 into geological formations to sequestrate CO2 and/or to enhance gas/oil production. In this study, the contact angles of Bentheimer sandstone/water/CO2 or flue gas have been evaluated experimentally using the captive-bubble technique in the pressure range from 0.2 to 15 MPa. The experiments were conducted using different compositions of aqueous phase with respect to CO2, i.e., unsaturated and fully saturated. It has been shown that a reliable contact-angle determination needs to be conducted using a pre-equilibrated aqueous phase to eliminate dissolution effects. In the fully saturated aqueous phase, the Bentheimer sandstone/water system is (and remains) water-wet even at high pressures against CO2 and/or flue gas. In these systems, the data of the stable contact angle demonstrate a strong dependence on the bubble size, which can be mainly explained by the gravity (buoyancy) effect on bubble shape. However, the surface nonideality and roughness have significant influence on the reliability of the contact-angle determination. The results of this study prove that in order to avoid the dependency of the contact angle on the bubble size in these systems, the effect of gravity (buoyancy) on bubble shape has to be considered by calculation of the Bond number; for systems characterized by Bond numbers less than 0.9, the influence of the bubble radius on the contact angle becomes insignificant. The experimental results show that, in contrast to quartz, the phase transition of CO2 from subcritical to supercritical has no effect on the wettability of the Bentheimer sandstone/water system, which originates from differences in the surface charges of quartz and Bentheimer sandstone. In an unsaturated system, two dissolution regimes are observed, which may be explained by density-driven natural convection and molecular diffusion.