an expansion valve for a vapour compression system , and a vapour compression system comprising such an expansion valve are disclosed . the expansion valve comprises a first valve member and a second valve member . the first valve member and the second valve member are arranged movably relative to each other , and the relative position of the first valve member and the second valve member determines an opening degree of the expansion valve . the first valve member and / or the second valve member is / are automatically movable in response to changes in a differential pressure across the expansion valve , the opening degree of the expansion valve thereby being automatically altered in response to changes in the differential pressure across the expansion valve . it is ensured that the opening degree of the expansion valve is automatically adjusted to the actual operating conditions , thereby optimising the efficiency of the vapour compression system . furthermore , this is obtained in a simple manner , without requiring complicated control of the valve .