The reaction of Y2O3 and Li2CO3 with NH4F to produce LiYF4 was studied by thermogravimetric and X-ray diffraction methods. NH4F reacts easily with Li2CO3 in a one-step exothermic process, Fluorination of yttrium oxide gives first YF3 . 1.5NH(3) which decomposes at 300-380 degrees C to YF3 + NH3. This process is exothermic. In the absence of excess NH4F, an amount of YOF is produced, in addition to YF3, as a product of the reaction of YF3 and unreacted Y2O3. This reaction is endothermic. In the ternary system NH4F-Li2CO3-Y2O3, the first reacts separately with each of the other two and the resulting mixture of simple yttrium and lithium fluorides is converted into LiYF4 at high temperatures.