In a method for purifying an oil (8)/water (42) mixture (40), the mixture (40) is fed to a container (48) having an ultrafiltration unit (52) with a membrane (30), and a protective material (50) is added to the mixture (40), which protective material, directly or after chemical reaction with the water from the mixture (40), as a film (62) on the membrane (30), keeps free oil (8) away from said membrane. Purified process water for extracting and/or refining of petroleum (8) is thus produced from process water (42) which is contaminated with petroleum (8). A purification device (64) substantially comprises a container (48) having an ultrafiltration unit (52) with a membrane (30), a feed line (28) to the container (48) and an addition device (27) for protective material (50) in the flow path (56) to the membrane (30).