According to the invention, there is described a method and a system which use a compressor (4) for filling CO2 into an air conditioning system (18) via an oil filter (5) and a gas cooler (6). CO2 may be pumped to an internal pressure container (11), and from there the air conditioning system (18) may be filled with CO2 in gas form via valves (15). The fill system may additionally include a heat exchanger (9), so that a temperature may be controlled in the pressure container (11) so that it does not exceed 31.8 C, whereby CO2 may be filled into the air conditioning system (18) in liquid form.