The Bureau of Mines is developing melting data for binary combinations of the alkaline earth fluorides and yttria to facilitate the formulation of slag compositions. The present report deals with the srf2-yso3 system for which liquidus and solidus temperatures and solid solubilities were determined. The information is presented as the phase diagram. The phase diagram was constructed from information obtained by ceramography, x-ray diffraction, thermal analysis, and electron probe microanalysis of small specimens encapsulated in platinum-20 percent rhodium or molybdenum ampoules. The data indicate a simple eutectic system with limited terminal solid solubilities. The eutectic temperature is 1,457 deg+/-5 deg c, and the eutectic composition is 4+/-1 mole- pct y2o3. Strontium fluoride containing up to 2 wt-pct o2 was studied to determine the effects of oxygen contamination. Oxygen levels in excess of 0.2 wt-pct resulted in eutectic formation between srf2 and sro. The eutectic composition is between 0.2 And 0.7 wt-pct o2, and the eutectic temeprature is 1,447 deg+/-5 deg c.