Abstract: A vapour compression refrigeration system includes a compressor (1) arranged to re-circulate refrigerant through a condenser (2), an expansion device (4) and an evaporator (5). To achieve rapid thermodynamically efficient defrosting of the evaporator, hot refrigerant from the condenser is stored in a defrost receiver (6) before passing through the expansion device (4). In a defrost phase, a valve arrangement (7-10) forms a closed defrost circuit connecting the evaporator (5) to the defrost receiver (6) via defrost valve (10) to allow hot fluid to pass from the defrost receiver to the evaporator and liquid refrigerant in the evaporator flows to the defrost receiver (6) via drain valve (9). In a pre-defrost phase, the valve arrangement closes the fluid input to the evaporator (5) and the compressor operates to partially evacuate the evaporator before the evaporator is connected to the defrost receiver, so that flash flooding of the evaporator with hot vapour occurs.