Ionic liquids (ILs) have been proposed as promising solvents for hydrocarbon separations, but designing an industrially attractive IL combining high solubility and low viscosity remains challenging. Here, we synthesized three new long-chain carboxylate ILs with asymmetric phosphonium cations that had relatively low viscosity and good thermal stability and exhibited very high solubility and excellent selectivity for hydrocarbons with different carbon numbers under ambient conditions. The solubilities of propane, ethane, methane, and nitrogen in these tributylethyl-phosphonium long-chain carboxylate ILs were determined at temperatures of 298.1 to 313.1 K and pressures of 20 to 150 kPa. The effects of molecular structure on the properties of ILs and their absorption performance were investigated. It was found that the introduction of an asymmetric cation with short alkyl chains significantly reduced the viscosity of carboxylate as, while an extension on the alkyl chain of carboxylate anions enhanced the solubility. At 298.1 K and 150 kPa, the solubilities of propane, ethane, and methane in tributylethylphosphonium stearate reach 0.408, 0.133, and 0.029 mmol/g with selectivities of propane/methane and methane/nitrogen up to 16.92 and 2.72, respectively. This study demonstrates the great potential of long-chain carboxylate ILs as novel solvents for separating light hydrocarbons.