A quantitative understanding of transport properties of ionic liquids such as viscosity and diffusivity is necessary to utilize their diverse chemistry and potential applications. Understanding structural effects of ionic liquids on their physico-chemical properties can help for their molecularly design for specific purposes. Many applications involving compressed gases, such as carbon dioxide and refrigerant gases have been proposed. This work presents the high-pressure viscosity measurements of pure imidazolium-based ionic liquids at three temperatures and pressures up to 126 MPa. The effects of the length of the alkyl group and the anion are presented. A combined Litovitz-Tait equation was utilized to describe the viscosity data with pressure and temperature and compared with literature data. The liquid phase viscosity and diffusivity are reported for several ionic liquids with compressed the hydrofluorocarbon gas, 1,1,1,2-tetrafluoroethane (R-134a) and carbon dioxide up to 30MPa. The dissolved gas significantly enhances the diffusivity and reduces the viscosity of the liquid phase.