The aim of this review is to provide some information on the gas absorption characters of ionic liquids and the chemical processes utilizing ionic liquid + gas systems. Ionic liquids are salts with the melting points being at or below ambient temperatures. They are unique solvents, of which the characteristics are non-volatile, non-flammable, electrically conductive, miscible with various chemicals, and so on. One of the promising utilizations of ionic liquids is gas absorption media. In order to use the ionic liquids for the chemical processes involved with gas absorption, the understanding of the gas absorption property and mechanism is of primarily importance. This review presents some fundamental data on the following systems: (1) CO₂ physical absorption; (2) CO₂ chemical absorption; (3) NH₃ absorption; (4) water absorption. First, the two-phase equilibria are shown for some typical ionic liquid + gas systems in each section. Then, the gas absorption mechanisms are discussed on the basis of the thermodynamic and spectroscopic analyses. Finally, the separation, reaction-separation, and/or heat-pump processes using ionic liquid are briefly explained.