The solubility of carbon dioxide (CO2) in five tetraalkylphosphonium superbase ionic liquids, namely the trihexyltetradecylphoshonium phenoxide, trihexyltetradecylphoshonium benzotriazolide, trihexyltetradecylphoshonium benzimidazolide, trihexyltetradecylphoshonium 1,2,3-triazolide and trihexyltetradecylphoshonium 1,2,4triazolide was studied experimentally under dry and wet conditions at 22 °C and at atmospheric pressure using a gravimetric saturation technique. The effects of anion structure and of the presence or absence of water in solution on the carbon dioxide solubility were then deduced from the data. 1 H and 13 C-NMR spectroscopy and ab initio calculations were also conducted to probe the interactions in these solutions, as carbon dioxide and water can compete in the ionic liquid structure during the absorption process. Additionally, the viscosity of selected superbase ionic liquids was measured under dry and wet conditions in the presence or absence of CO2 to evaluate their practical application in carbon dioxide capture processes. Finally, the recyclability of the trihexyltetradecylphoshonium 1,2,4-triazolide under dry and wet conditions was determined to probe the ability of selected solvents to solubilize chemically a high concentration of carbon dioxide and then release it within a low energy demand process.