In the current work, a series of phosphonium-based monocationic and dicationic ionic liquids (trihexyltetradecylphosphonium dioctylsulfosuccinate, P6,6,6,14DOSS; trioctyltetradecylphosphonium dioctylsulfosuccinate, P8,8,8,14DOSS; 1,6-bis(trioctylphosphonium)hexane dioctylsulfosuccinate, P8,8,8C6P8,8,8 DOSS2; and 1,10-bis(trioctylphosphonium)decane dioctylsulfosuccinate, P8,8,8C10P8,8,8DOSS2) were prepared, and their characterization was done using NMR and elemental analysis. Their physiochemical properties (thermal stability, viscosity, density, and refractive index) and CO2 solubility (at pressures 1, 5, 10, 15, and 20 bar) were investigated. The cation and alkyl chain effects on the properties and CO2 capturing efficiency were discussed. The thermodynamic functions of solvation were calculated using the variation of Henry's law constants with temperature. © 2018 American Chemical Society.