A cryogenic propellant submitted to heat load during long duration space missions tends to vaporize to such an extent that the resulting pressure rise must be controlled to prevent storage failure. The Thermodynamic Vent System TVS), one of the possible control strategies, has been investigated using on-ground experiments with NOV EC 1230 as substitution fluid. Results obtained for self-pressurization and TVS control phases have been reported in a previous work. The unexpected inverse thermal stratification observed during these experiments is analyzed in the present work and related to the influence of non-condensable gases. Non-condensable gases, present inside the tank in the form of nitrogen - 10 times lighter than the substitution fluid vapor - generate a concentration stratification in the ullage. Assuming the NOV EC 1230 remains at saturation in the whole ullage, the density stratification which results from this concentration stratification can explain the observed inverse thermal stratification.