The Bureau of Mines investigated the complete ternary system k2hff6- k2zrf6-1.25 Weight-percent hf at 40 deg c. In addition, solubility curves have been established at 25 deg, 30 deg, 50 deg, 60 deg, and 70 deg c. These solubility data implemented a study of hafnium enrichment in feed solutions for liquid-liquid extraction processes. Selected k2hff6-k2zrf6-1.25 Weight-percent hf mixtures were equilibrated in a constant temperature bath. Compositions of the equilibrated solutions and wet residues were established from combined hafnium and zirconium oxide and total fluorine analyses. All solubility curves were essentially parallel on triangular coordinates. At 40 deg c, the solid phase of the k2hff6-k2zrf6-1.25 percent hf system is a series of solid solution hydrates with k2hff6.H2o and k2zrf6.3H2o segments existing at the respective extremes. Since this system contains k2hff6.H2o and k2zrf6.3H2o segments, it is not ideal for simple separation of high-purity k2hff6 from k2hff6-k2zrf6 mixtures by recrystallization. However, it would be useful for producing hafnium-enriched feed solutions from zircon concentrates for liquid-liquid extraction of hafnium from zirconium.