Solubility of trioctylmethylammonium chloride (TOMAC) in supercritical carbon dioxide (SCCO2) was measured at 313, 323 and 333 K with pressure ranging from 10 to 30 MPa. Solubilities (in mole fraction) ranged from 0.5 × 10−5 to 12.7 × 10−5 in the investigated region. Influence of co-solvents on the solubility of TOMAC was studied at 313 and 323 K. Among the four models employed for correlating solubility of TOMAC in neat SCCO2, association theory based on van Laar activity coefficient model resulted in average deviation of 2%, while use of other models resulted within 10%. Solubility data were found to be self-consistent based on the Mendez-Teja model. Two new models were developed for the ternary system (SCCO2 + co-solvent + liquid solute) based upon association theory along with the Wilson and van Laar activity coefficient models. These models successfully correlated solubility of TOMAC in SCCO2 + co-solvent with an average deviation of <8%. © 2018 Elsevier B.V.