Solubilities are reported for methane, ethane, ethylene, and propane in trimethyloctylphosphonium bis(2,4,4-trimethylpentyl) phosphinate [P8111] [TMPP] from 299 to 323 K up to 4 MPa. Consistent with prediction of COSMO-RS, [P8111] [TMPP] shows large solubilities for all four hydrocarbons. Contrary to solubility data in conventional ionic liquids, in [P8111] [TMPP] the solubility for ethane is higher than that for ethylene. When compared with solubilities in other similar phosphonium-based ionic liquids, the cation has only a small influence on the solubilities of small hydrocarbons. Ionic liquid [P8111][TMPP] may be useful for storage or separation of small hydrocarbons.