Ionic liquid (IL) is a molten salt with melting point or glass transition temperature below 100 °C. In the past few years, ILs have been intensively promoted as designer media for various chemical processes. By selecting an appropriate combination of the cation and anion, some unique physicochemical properties including, low vapor pressure characteristic, polarity–miscibility, electrochemical window, liquidus range, viscosity, and thermal stability can be tuned for a specific task. Various room temperature ionic liquids (RTILs), classical and task-specific ones have been prepared and their potential applications for CO2 capture and for extractive separation of light alkynes from olefins have been evaluated. From detail investigations on the solubilities of CO2, ethylene, propylene, acetylene, and propyne in wide variety of RTILs, solubility–selectivity trends, thermodynamics of absorptions and molecular models were derived and plausible solute–solvent interactions were proposed. Based on those findings, appropriate RTILs for the aforementioned tasks were advised.