In order to further understand the dependence of the refractive index on composition, a systematic investigation of the relationship between these quantities has been undertaken. Starting with well known stable quinary glass compositions containing zirconium tetrafluoride, the effects of dopants such as alkali ions and PbF2 on the refractive index have been investigated. In addition, thermal analysis was carried out to ascertain the extent to which glass stability had been affected by the compositional changes. A refractive index difference, Δn, of up to 0.02 and numerical aperture (NA) of 0.23 was obtained at compositions for which the difference in glass transition temperature, ΔTg, was only 1°C. The molar refractivity for stable glass systems showed a good correlation with theoretically predicted results, except for systems containing LiF.