Ionic Liquids (ILs) are considered as alternative solvents for the separation of CO2 from flue gas due mainly to their CO2 affinity and thermal stability. The cation architecture in a matrix of ammonium and mostly phosphonium-based ILs with 2-cyanopyrrolide as the anion to evaluate its impact on gravimetric CO2 absorption capacity, viscosity, and thermal stability and the three fundamental properties vital for application realization are systematically investigated. Among the investigated ILs, [P2,2,2,8][2-CNpyr] demonstrated the lowest viscosity, 95 cP at 40 degrees C, and highest CO2 uptake, 114 mg CO2 per g IL at 40 degrees C. Combined effects of asymmetry and the optimized chain lengths also resulted in improved thermal stability for [P2,2,2,8][2-CNpyr], with a mass loss rate of 1.35 x 10(-6) g h(-1) (0.0067 mass % h(-1)) at 80 degrees C. (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 2280-2285, 2015