Operation of an oil-water separator or other liquid mixture separator is monitored. The separator uses a vessel through which a mixture of different liquid components flows horizontally. A plurality of ultrasound transducers is provided on a wall of the vessel at different heights at a common stage along the direction of the horizontal flow. Ultrasound at a plurality of different ultrasound wavelengths is transmitted through the vessel between transducers and ultrasound arriving transducers is detected. Parameter values of a model are fitted to the detection. A model is used that relates height dependent properties of liquid in the vessel to properties of the detected ultrasound transmission between transceivers, the model comprising at least a fittable parameter of a height dependent droplet size distribution in a least a sub-range of a height of liquid in the vessel. The model relating the droplet size distribution to ultrasound wavelength dependence of the properties of ultrasound transmission. The measured droplet size distribution may be used to determine the route of ultrasound paths and to control demulsification measures.