A process for monitoring an air conditioning system of a railway vehicle , said air conditioning system being of the type with steam compression and including a motorized compressor , a condenser , an evaporator and an expander connected to one another by a refrigerant circuit , said air conditioning system being installed to provide air conditioning for a zone located inside the railway vehicle, said process being characterized in that it comprises steps consisting of: a) after starting the air conditioning system, measuring physical properties relative to the refrigerant and an outside flow of air exchanging heat with the evaporator or with the condenser of the air conditioning system and to the mechanical work ( Ẇ compr ) supplied by the compressor to the refrigerant, these measurements being done using sensors equipping the air conditioning system, the mechanical work being determined from the electrical voltage U between the electrical phases of an electric motor of the compressor, from the intensity of the electrical current I supplying said motor and from the power factor cos φ of said motor using the formula W ̇ compr = 3 UIcos ⁢ φ ; b) calculating , using an electronic calculator of the railway vehicle, the volumetric efficiency and the isentropic efficiency of the compressor, from the physical properties measured during step a); c) calculating , using the electronic calculator, a health index ( HI comp ) of the compressor, from the volumetric efficiency and the isentropic efficiency calculated during step b) and from the mechanical work supplied by the compressor to the refrigerant; d) comparing , using the electronic calculator, the health index calculated during step c) with a reference value, the compressor then being considered no longer to work in a satisfactory manner if the health index is below the reference value, and otherwise being considered to work in a satisfactory manner. a) after starting the air conditioning system, measuring physical properties relative to the refrigerant and an outside flow of air exchanging heat with the evaporator or with the condenser of the air conditioning system and to the mechanical work ( Ẇ compr ) supplied by the compressor to the refrigerant, these measurements being done using sensors equipping the air conditioning system, the mechanical work being determined from the electrical voltage U between the electrical phases of an electric motor of the compressor, from the intensity of the electrical current I supplying said motor and from the power factor cos φ of said motor using the formula W ̇ compr = 3 UIcos ⁢ φ ; b) calculating , using an electronic calculator of the railway vehicle, the volumetric efficiency and the isentropic efficiency of the compressor, from the physical properties measured during step a); c) calculating , using the electronic calculator, a health index ( HI comp ) of the compressor, from the volumetric efficiency and the isentropic efficiency calculated during step b) and from the mechanical work supplied by the compressor to the refrigerant; d) comparing , using the electronic calculator, the health index calculated during step c) with a reference value, the compressor then being considered no longer to work in a satisfactory manner if the health index is below the reference value, and otherwise being considered to work in a satisfactory manner. <EOS>