There is a role for GaF3 in both the core and cladding of heavy metal fluoride glass fibres. Substitution for AlF3 in the core can prevent tensile stresses occurring in the cladding during cooling, by increasing the thermal expansion coefficient in the core, with no measurable effect on the refractive index. When substituted for BaF2 in the cladding, the refractive index is decreased and the thermal expansion coefficient is increased. Use of GaF3 in conjunction with HfF4 gives a useful additional degree of freedom in the design of the cladding glass. In combination with GaF3 for AlF3 substitution in the core glass, this substitution allows a higher index difference to be achieved in a compatible pair of compositions for a given concentration of PbF2 in the core.