Three ionic liquids {butyl-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide [N-1114][BTA], 1-methyl-1-propyl-piperidinium bis(trifluoromethylsulfonyl)imide [PMPip][BTA], and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][Tf]} and two heat-transfer oils [dibenzyltoluene (DBT) and polydimethylsiloxane (trade name X-BF)] were evaluated for use in the three-phase methanation and the biogas purification processes. The density, viscosity, and surface tension of these liquids were measured and modeled as a function of the temperature. The solubilities of H-2, CO, CO2, and CH4 in these five liquids were also obtained under different pressures and temperatures. Additionally, the criteria required for each of the two processes considered were identified: the three-phase methanation process requires a thermally stable liquid with a low vapor pressure and a high H-2, CO2 and CO solubility, while the biogas purification process requires a highly selective CO2 solubility liquid at ambient temperature. From the evaluation of both the experimental data and the process requirements, the most suitable liquid for each of the aforementioned processes was identified. For the three-phase methanation process, the two ionic liquids [N-1114][BTA] and [PMPip][BTA] and the two heat-transfer oils DBT and X-BF met the minimum requirements, while [EMIM][Tf] showed promising potential for the biogas purification process.