An apparatus (10) for measuring, in particular, the amount of oil present in a quantity of water. The apparatus comprises a measurement chamber (12) having an optical window (18) through which an excitation signal may be transmitted and fluorescent light may be detected. The apparatus further includes an ultrasonic transducer (34) coupled to the measurement chamber and having a pair of channels (44,45) formed therein, the channels opening onto the measurement window (18). A respective light guide (28) is inserted into each channel, one light guide being arranged to deliver the excitation signal into the chamber through the measurement window, the other being arranged to carry fluorescent light from the chamber.