Ionic liquids are a new kind of solvent used in "green chemistry" for their non-existent vapor pressure. Numerous chemical reactions can be carried out in these new salts which are liquid at room temperature. In some processes there is a separation after the reaction, or some reactants are in a gas phase. Phase equilibria and mass transfer data are necessary to optimize these stages. Solubility and, for the first time, diffusivity of carbon dioxide and ethylene in 1-butyl-3-methylimidazolium hexafluoro-phosphate are presented in this chapter. The diffusivity showed a different trend than expected with the viscosities and the molar volumes of the solutes.