The presence of optical radiation outside the core of a step index fibre - the so called evanescent wave - offers a technique for chemical sensing by attenuated total interval reflection spectroscopy (ATR). The penetration of the evanescent wave power in the sensing region is dependent upon the modal structure in the fibre core. In this thesis a method for measuring the variation of modal power distribution with distance along the fibre using a CCD camera is described and used for large diameter plastic clad silica fibre (PCS). Both linear and two dimensional array cameras are used to profile the optical power distribution in the fibre core and the results obtained are discussed in terms of diffusion models. The suitability of such fibre for ATR spectroscopy is evaluated in light of the experimental results.