A study of fluorescence excited by the evanescent wave (E.W.) of guided radiation in an optical fibre is reported. E.W. excitation of fluorescence is important because of its application in novel optical sensors for biomedical and chemical parameters. An experimental system whose design is based on the theory of evanescent wave (E.W.) excitation of fluorescence is described. Several chemical immobilisation techniques for the attaching of fluorescent dye molecules onto the surface of an unclad optical fibre core were investigated and were found to be unsatisfactory. These techniques when examined using evanescent wave excitation lacked consistency with regard to their optical emission spectra, peak wavelength and intensity of fluorescence. A novel technique to coat an unclad optical fibre core surface with a porous glass coating (a Sol-gel) which was doped with fluorescent dye was investigated and found to have potential in the area of optical fibre sensors. Fluorescent dyes such as fluorescein, fluorescein isothyocyanate and rhodamine 6G were used as initial dopants within the Sol-gel matrix. Using the technique of evanescent wave excitation the fluorescent dyes were found to exhibit emission spectra comparable to that of bulk excitation. A pH probe was constructed using a fluorescein-doped Sol-gel coating around a short length (90mm) of unclad optical fibre. It was found to have a pH sensitivity in the range of pH2 to pH7 and a response time of approximately 5s.