During the present decade, many experimental results on phase equilibria phenomena such as cosolvency, miscibility windows, and two-phase Ig holes in three-phase Ilg surfaces (non-critical Ig holes in the critical surface) in ternary systems were obtained by various research groups. It is the aim of this paper to collect and sort these results in order to present a summary and a status report of experimental and theoretical research in this field. The emphasis will be on ternary systems of near- and supercritical carbon dioxide with one polar and one non-polar component (here mostly a 2-alkanol and an alkane). It will be demonstrated how cosolvency effects influence the appearance of miscibility windows as well as the occurrence of two-phase Ig holes in the three-phase Ilg surface and how these phenomena are related to each other.