The separation of water from diesel fuel is very important for safety, ecological, and economic reasons, as otherwise it can lead to lower combustion efficiency and engine problems. In addition, the free water from ultra-low-sulfur diesel (ULSD) can only be insufficiently separated with the classic separation systems. To solve this problem, a membrane process with different organic/inorganic membranes for the selective separation of water droplets from ULSD and an innovative water-in-oil online sensor were developed.