Abstract: Liquid, particularly leaked oil or other hydrophobic liquid, is detected by a sensor. This preferably has a hydrophobic membrane (G) that takes up the oil selectively, a radiation source (A) that beams radiation at an interface between the membrane (G) and a window (F), and a radiation detector (H) that receives radiation resulting from interaction (such as reflection, scattering or fluorescence) of the input radiation with the liquid-containing membrane (G). The detector may employ a spectrofluorimeter (H) whose output can be used to characterise the liquid.