In this work, the inter-facial tension (IFT) of a two-phase system consisting of compressed carbon dioxide (CO2) and two imidazolium-based ionic liquids (ILs), 1-ethyl-3-methylimidazolium 2-(2-ethoxyethoxy)ethylsulfate (1) and 1-butyl-3-methylimidazolium 2-(2-methoxyethoxy)ethylsulfate (2), was measured in the pressure range of (0.1 to 20) MPa at (280, 300, and 320) K. The quasi-static procedure of the pendant drop method was used. The IFT follows the general trend, that is, receding values at increasing pressure and decreasing temperature. The formation of bubbles in the IL phase and visible phase instabilities during mutual saturation of the phases suggest a high enthalpy of mixing. The viscosity of substance 2 significantly changed under CO2 pressure. No chemical reaction is visible in the IR spectrum. Therefore, this increase in viscosity results from the formation of a new solid phase.