In literature, equations exist to evaluate the thermodynamic properties of gases and mixtures, like the dew point curves. However, the knowledge of fluoronitrile mixtures is still limited and these equations need to be calibrated with experimental data. Within this scope, experimental tests have been conducted with three types of C4FN mixtures. The tests have been performed in a climatic chamber cooling down to -40°C and a GIS compartment of about 50 L that has been equipped with different measurement sensors to monitor the temperature and the pressure of the mixture. Six different pressures for each mixture, from 5 to 10 bar abs have been investigated. The liquefaction points have been determined by decreasing the temperature until the emergence of the first drops of liquid, identified by a decreasing of the pressure with a numerical methodology. The conducted analyses have permitted to increase the knowledge on the liquefaction point of the C4FN mixtures and to investigate the effect of O2 in the liquefaction behaviour. With the consistent number of data acquired during the test campaign, the experimental measurements have been fitted to obtain an equation that allows, within a certain validity range, the determination of the liquefaction points of mixtures for different molar ratios of C4FN, O2 and CO2 and different pressures. The equations of state for the fluoronitrile mixtures, following the Peng-Robinson equation, have been equally determined from the experimental data. The derived equations are very interesting for the practical application of the fluoronitrile mixtures in GIS and allows the evaluation of the liquefaction points at different densities even if the molar quantity of C4FN is changing all along the lifetime, e.g. differential permeation or refilling. It helps, for the operational point of view, to estimate the need or not for a maintenance of the GIS to recover good insulation characteristics.