A method for controlling a vapour compression system during start-up, the vapour compression system comprising a compressor, a condenser, an expansion device having a variable opening degree, and an evaporator arranged along a refrigerant path, the method comprising the steps of: - starting operation of the vapour compression system, - monitoring a first temperature, T 1, of refrigerant entering the evaporator, - monitoring a second temperature, T 2, of refrigerant leaving the evaporator, characterized by the steps of: - deriving a first rate of change, ΔT 1, of the first temperature, and a second rate of change, ΔT 2, of the second temperature, - comparing the first rate of change, ΔT 1, to the second rate of change, ΔT 2, - based on the comparing step, determining a refrigerant filling state of the evaporator, wherein it is determined that the evaporator is full or almost full when the first rate of change ΔT 1 is substantially identical to the second rate of change ΔTt 2 and controlling the opening degree of the expansion device according to a first control strategy in the case that it is determined that the evaporator is full or almost full, and controlling the opening degree of the expansion device according to a second control strategy in the case that it is determined that the evaporator is not full, wherein the first control strategy comprises the step of gradually decreasing the opening degree of the expansion device, and the second control strategy comprises the step of gradually increasing the opening degree of the expansion device, further comprising the steps of: - monitoring a difference between the first temperature, T 1, and the second temperature, T 2, during the step of gradually decreasing the opening degree of the expansion device, and - discontinuing decreasing the opening degree of the expansion device in the case that the difference between the first temperature, T 1, and the second temperature, T 2, exceeds a predetermined threshold value - monitoring the second rate of change, ΔT 2, during the step of gradually increasing the opening degree of the expansion device, and - discontinuing increasing the opening degree of the expansion device in the case that the numerical value of the second rate of change, ΔT 2, exceeds a predetermined threshold value. - starting operation of the vapour compression system, - monitoring a first temperature, T 1, of refrigerant entering the evaporator, - monitoring a second temperature, T 2, of refrigerant leaving the evaporator, characterized by the steps of: - deriving a first rate of change, ΔT 1, of the first temperature, and a second rate of change, ΔT 2, of the second temperature, - comparing the first rate of change, ΔT 1, to the second rate of change, ΔT 2, - based on the comparing step, determining a refrigerant filling state of the evaporator, wherein it is determined that the evaporator is full or almost full when the first rate of change ΔT 1 is substantially identical to the second rate of change ΔTt 2 and - monitoring a difference between the first temperature, T 1, and the second temperature, T 2, during the step of gradually decreasing the opening degree of the expansion device, and - discontinuing decreasing the opening degree of the expansion device in the case that the difference between the first temperature, T 1, and the second temperature, T 2, exceeds a predetermined threshold value - monitoring the second rate of change, ΔT 2, during the step of gradually increasing the opening degree of the expansion device, and - discontinuing increasing the opening degree of the expansion device in the case that the numerical value of the second rate of change, ΔT 2, exceeds a predetermined threshold value.