A monitor for monitoring the concentration of oxygen in a fuel or in an ullage over a fuel comprises (i) a sensing element comprising a luminescent substance comprising a luminophore and a support in which the luminophore is covalently bound to the support, (ii) a light source arranged to irradiate the sensing element with light, and (iii) a photosensor arranged to detect light emitted from the luminescent substance. The luminescent substance may be, for example, a platinum porphyrin covalently bound to silica.