A liquid-core fibre-optic waveguide comprising a hollow vitreous fibre filled with a liquid wherein the liquid is substantially a single compound, a mixture of compounds, or a single compound or a mixture of compounds dissolved in or diluted with a solvent, the said compound or compounds being selected from the class of aliphatic compounds which contain from three to seven carbon atoms per molecule and are fully halogenated with fluorine, chlorine, bromine or any combination thereof, such that the liquid is liquid at 25* centigrade, has a refractive index greater than that of the vitreous fibre and has minimal optical absorption and scattering throughout at least a substantial part of the spectral range from 0.5 to 2.0 microns. The preferred vitreous fibre is soda-lime borosilicate glass and the preferred liquid is hexachloro-1,3-butadiene. Alternative liquids are hexachloropropene and 1,4-difluoroctachlorobutane; a mixture of any two or all three of these compounds may be used in a silica or quartz fibre. Heating or the addition of a solvent may be used to facilitate the filling of the fibre with the liquid. The preferred solvents are carbon tetrachloride and tetrachloroethylene but bromo-trichloromethane or tribromofluoromethane could alternatively be used.