A method and a sensor (1) for oil-in-water measurement, in particular for the oil industry. In an electric measuring cell (7, 12) a capacitance is measured as a measure of an oil concentration in the water flowing through or in the accumulation filter (16, 16a, 16b). In order to reduce the maintenance requirement, the measuring cell (7, 12) is automatically calibrated at recurring time intervals with clean water (FIG. 1) and/or flushed with water, possibly oil-contaminated, in the back-flushing direction (FIG. 2). Advantages of the invention are reduced signal drift, improved long-term reliability and a long service life without monitoring or filter exchange. Important exemplary embodiments relate to: a measuring cell (7, 12) with a long measuring and short flushing/calibration cycle, a measuring cell (7, 12) with a plurality of accumulation filters (16a, 16b) whose measuring cycles supplement one another in time, a cross-flow filter (2) for oil enrichment upstream of the measuring cell (7, 12) and a compact, pressure-resistant cylindrical ring-type filter (13) made from porous ceramic (16, 16a, 16b) with measuring and flushing connections radially (17a, 17b, 18a, 18b) and axially (17c, 18c).