1. A system ( 30 , 30 ′) for controlling an air-conditioning system ( 1 ) of the passenger compartment of a vehicle ( 1 a ) provided with an evaporator ( 13 ) and a compressor ( 18 ) coupled to said evaporator ( 13 ), comprising a control unit ( 33 ) configured to switch an operating condition of said compressor ( 18 ) when the temperature at outlet from said evaporator ( 13 ) has a pre-set relation with a threshold temperature (T thresh ), characterized in that said control unit ( 33 ) is further configured to vary the value of said threshold temperature (T thresh ) as a function, in at least certain operating conditions, of a set-point temperature (T sp ), required by a user of said air-conditioning system ( 1 ).