Abstract: In the operation of a refrigeration system comprising a compressor, a condensor, an expansion valve and an evaporator which form a circulation circuit containing NH.sub.3 refrigerant as a first medium, the compressor being lubricated by a second medium, oil, part of the oil quantity being continuously discharged from the compressor together with compressed NH.sub.3 -gas into the circulation circuit for recirculation of oil to the compressor, continuously or batchwise, from the low pressure region of the circulation circuit, that is, the region located between the expansion valve and the compressor, the part of the oil transferred to the low pressure region (5, 6, 8, 9, 10, 11) of the circulation circuit is contacted with a third medium, added to the circulation circuit and which is substantially insoluble in liquid NH.sub.