A device for condensing volatile organic compounds (VOC) from a storage or transport tank (4) into oil from the same or another storage or transport tank (4) via a downcomer (6), where the upper part (8) of the downcomer (6), which upper part has a cross sectional area that is essentially unreduced, is connected to a gas pipe (14), the gas pipe (14) communicating with the upper part (16) of the storage or transport tank (4), and where the upper part (8) of the downcomer (6) is arranged at a sufficient height above the upper part (16) of the storage or transport tank (4) to cause an inflow of gas from the upper part (16) of the storage or transport tank (4) due to the underpressure created in the upper part (8) of the downcomer (6) when oil flows down through the downcomer (6).