During this work, an original study of the CO 2 and SO 2 competitive gas absorption in three superbase ionic liquids (ILs), namely, trihexyltetradecylphosphonium 1,2,4-triazolide ([P 66614 ]-[124Triz]), and trihexyltetradecylphoshonium benzimidazolide ([P 66614 ][Benzim]), is reported for the first time. To initiate such a comprehensive study, the CO 2 and SO 2 mixed gas solubility in selected ILs was determined by using an original and accurate dynamic method coupled with mass spectrometry after several absorption and desorption cycles. This method has been validated by comparing the gravimetric uptake of CO 2 with the mass spectrometry data using trihexyltetradecylphosphonium benzotriazolide, 1,2,4-triazolide, and benzimidazolide ILs and shown to be consistent within 10% in mole ratio units. Solubility results clearly show that the presence of SO 2 in the gas stream decreases the CO 2 capture capability of the investigated ILs. Furthermore, the viscosity, chemical analysis (water content and sulfur content), and spectroscopic data ( 1 H NMR, 13 C NMR, attenuated total reflectance-infrared, and X-ray photoelectron spectroscopy) changes before and after absorption−desorption of the gases were determined and depicted to truly understand the reaction mechanism that occurs in the liquid phase, highlighting a clear competition between the SO 2 versus CO 2 chemical reaction and selected superbase ILs.