Cosolvency effects and miscibility windows were studied in selected ternary systems containing a supercritical solvent (CO2), a 1-alkanol (1-nonanol to 1-undecanol) and an n-alkane as the third component (n-pentadecane to n-octadecane) according to the analytical method. In the present experiments the ranges of temperature and pressure were 288 K to 393 K and 7.5 MPa to 100 MPa respectively. A new technique (so-called quasi-synthetic method) is described to determine critical parameters of binary mixtures from synthetic measurements performed with the gaseous component being the pressurizing medium. It is the aim of the present investigations to give a quantitative description of the cosolvency effect as a function of temperature and of the chain lengths of the 1-alkanol and the n-alkane respectively, as a continuation of the systematic investigations of Kordikowski and Schneider (1993) as well as of Pohler and Schneider (1995). In order to compare systems with or without a pressure minimum on the ternary critical line at a constant temperature, a mean cosolvency effect was introduced, being a measure for improved solubility in ternary systems which expresses itself by a positive curvature of the ternary critical line at constant temperature.