We have studied electron paramagnetic resonance spectra of europium- and gadolinium-activated ZBLAN-type fluorohafnate glasses, using the composition 58HfF 4 · 20BaF 2 · 2LaF 3 · 3AlF 3 · 17NaF as an example. The ratio of the concentration of free Eu 2+ and Gd 3+ ions to that of ions in clusters has been quantitatively evaluated for the first time. The percentage of free ions has been shown to increase significantly with decreasing activator concentration. At activator concentrations of 1.25 mol % EuF 2 and 1 mol % GdF 3 , the activator ions predominantly form clusters and only a small fraction of Eu 2+ and Gd 3+ are present as individual ions, whereas at 0.1 mol % EuF 2 the concentration of free ions is comparable to that of ions in the clusters.