Heavy metal fluoride glasses of varying alkali metal fluoride have been studied by positron annihilation lifetime spectroscopy. The alkali series 53ZrF4 · (40 - x)BaF2 · 4LaF3 · 3AlF3 · xLiF and 53ZrF4 · (40 - x)BaF2 · 4LaF3 · 3AlF3 · xCsF are compared with the mixed alkali series 53ZrF4 · 20BaF2 · 4LaF3 · 4LaF3 · 3AlF3 · (20 - x)LiF · xCsF. The mean lifetime and refractive index are non-linear functions of dopant concentration, (40 - x)BaF2 · xLiF, in the Li-doped alkali series as compared with the linear dependence of mean lifetime and refractive index on the dopant concentration, (20 - x)LiF · xCsF or (40 - x)BaF2 · xCsF found in the mixed alkali and Cs-doped alkali series. The Li-doped alkali series spectra can be best fitted by a single lifetime, while the mixed alkali series and Cs doped alkali series spectra can be best fitted with two lifetime components. It is proposed that CsF doping increases the molar volume of the glass and produces sites of reduced electron density available for positron trapping.