A non-azeotropic fluid mixture for air conditioning apparatus characterized because the mixture comprises the following refrigerant fluids (gases) in given proportions: 2-Methylpropane (Isobutane) CH(CH3)3 (R-600a), Pentafluoroethane (R-125), Tetrafluoroethane (R-134a) Difluoromethane (R-32). The composition of the mixture contains at least one polymerization inhibitor agent (hydroquinone, benzotriazole, dimethyl-t-butylphenol).