Reported are the solubility, diffusivity, and permeability data for various gases in five phosphonium-based ionic liquids at 30 degrees C, as determined with a lag-time technique. The ionic liquids have a viscosity range of 200-3000 cP. The gas solubilities and diffusivities of the phosphonium-based ionic liquids are of the same magnitude as the gas solubilities for the more familiar imidazolium-based liquids. The gas diffusivity appears to be inversely proportional to the viscosity with an average power of 0.35 for the phosphonium-based ionic liquids. This is in contrast to the power of 0.6 for the imidazolium-based ionic liquids, suggesting that the viscosity-diffusivity relationship varies for different classes of ionic liquids. Despite the generally higher viscosities of the phosphonium-based RTILs compared to the imidazolium-based RTILs, the similarity in thermodynamic and transport properties allows the consideration of the phosphonium-based RTILs as low-cost alternatives for reaction media or separation agents. The gases tested were carbon dioxide, ethylene, propylene, butene, and 1,3-butadiene. Also reported are the permeabilities of methane, nitrogen, and oxygen.