Optical waveguide operation in the mid-i.r. waveband requires the use of alternative materials to those conventionally based upon silica, this being a necessary consequence of fundamental absorption mechanisms; in selecting such materials the anionic component is the primary consideration. For high performance waveguides it is necessary to have as nearly coincident zero material dispersion and minimum loss wavelengths as possible in order to utilise the i.r. low loss potential and also to maximise bandwidths for monomode operation. Short distance power or image transfer applications are dependent solely upon the material minimum loss wavelength.