Fibre optic sensing is a growing technology in analytical chemistry. Scattering, absorbance, reflectance and luminescence spectroscopic measurements have been made using fibre waveguides. Fluorescence is particularly suited for use in fibre sensing because of its sensitivity and versatility, and the ease with which this type of measurement may be implemented with a single fibre optic.The probe configuration used in this study for pH measurement consists of fluorescein isothiocyanate (FITC) covalently bonded to a porous glass bead. This bead is attached to a single, multi-mode optical fibre, which conducts both the excitation and emission radiations. The increase in fluorescence intensity, as the acid form of the immobilised FITC is converted to its basic form, is related to the pH. The dynamic range of this miniature probe is pH 3–7 and this can be extended by decreasing the dye loading. The response time is between 20 and 35 s, depending on the capacity.