The performance of masking reagents, viz., ethylenediamine-N,N,N′,N′-tetraacetic acid, trans-1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid, diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) and triethylenetetramine-N,N,N′,N″,N″′,N″′-hexaacetic acid(TTHA), was studied for the determination of fluoride in the presence of Hf^IV and Th^IV using an ion-selective electrode. Hafnium(IV) was effectively masked using an excess of DTPA at pH 6. TTHA was the best reagent for masking Th^IV, while DTPA provided an alternative for fluoride concentrations of <1 × 10^–5 mol dm^–3. The masking reaction was rapid and contamination from membrane-dissolution was negligible. Fluoride contents in raw materials of fluoride glasses, viz., ZrF₄, HfF₄, and ThF₄, were determined.