The liquid-liquid-vapor (11g) partial miscibility behavior of the mixture carbon dioxide + 1-methylnaphthalene + acetone was experimentally studied by the use of a visual cell (stoichiometric) technique. Phase compositions and molar volumes of the two liquid phases in equilibrium are reported as functions of temperature and pressure within the llg region. The addition of the co-solvent acetone extends the three-phase llg region in pressure-temperature space from the binary carbon dioxide + n -1-methylnaphthalene llg locus downward in pressure. towards the vapor pressure curve of the co-solvent acetone. The three-phase region is bounded from above by an upper critical end point (UCEP) (1-1=g) locus. The presence of the co-solvent acetone significantly enhances the amount of the solute 1-methylnaphthalene taken up by the second (carbon-dioxide-rich) liquid phase.