A chemical sensing probe comprises an optical fiber (2) or may be mounted on an optical fiber (2). The probe has a chemically sensitive measuring material, such as a cross-linked polymer hydrogel (8), which exhibits a change in volume and/or a change in refractive index in the presence of a given chemical. The change in volume and/or refractive index gives a change in an optical path length through the probe which can be measured interferometrically.
