The use of multimode optical fibre as an intrinsic chemical sensor, with application in on-line analysis in the process industry, is described. The technique of attenuated total reflection spectroscopy is applied to the unclad section of the fibre which is in contact with the chemical being detected. A model based on selective mode propagation is developed to relate the evanescent absorption coefficient of the fibre probe to the bulk absorption coefficient of the absorbing species. An experimental system was constructed to verify the theoretical model. Evanescent wave absorption in an aqueous dye solution was performed using multimode fused silica fibre which was unclad at the sensing region. In order to produce modes close to cutoff in the sensing region, tunnelling modes were launched into the clad lead-in fibre. The measured evanescent absorbance of the dye solution was found to vary linearly with the exposed core length and to exhibit a square root dependence on concentration. The former effect is predicted from theory while the latter is attributed to adsorption on the core surface which obeys a Debye-Huckel-type concentration dependence. In adddition, a concentration enhancement of two orders of magnitude was observed due to this adsorption. Because the adsorption process is of an irreversible nature, the potential for evanescent wave spectroscopy on fused silica fibre is severly limited for ionic solutions. The effect may be exploited, however, in disposable probes to give increased sensitivity due to the enhanced surface concentrations.