A study of zirconium fluoride fibre for evanescent wave gas sensing in the 3 to 4(j.m region of the optical spectrum is presented. A range of fibre types and methods of evanescent field access is investigated both experimentally and theoretically. The methods of evanescent wave access include side-polishing of all glass monomode, tapering of all glass monomode and the use of porous clad multimode optical fibre. Preparation of zirconium fluoride fibres for sensing requires great care and the polishing and tapering techniques presented are substantially different from those currently used for Silica fibre. A number of experimental systems have been employed, using both broadband and laser sources, with thermoelectrically and liquid nitrogen cooled detectors, side-polished and tapered monomode fibre sensors are shown to have very low sensitivty to gaseous analytes. The use of a novel mid-IR LED source is also reported. The effect of analyte refractive index on evanescent wave sensitivity is discussed. It is shown that the larger the refractive index difference between fibre and analyte the greater the optical mode confinement. This results in a reduced evanescent wave penetration into the analyte. Methods are presented to overcome this mode confinement by use of porous claddings and overlayers. In particular, the use of a teflon clad fibre and sol-gel porous glass structures is proposed. A viable gas sensor based on Teflon clad fibre with a LED source is presented. This evanescent wave gas sensor has a limit of detection per unit length better than 1 % propane in air.