Nanocrystals of yttrium lutetium phosphates of the general formula Y1−x LuxPO4 ¡¤ nH2O are synthesized. The temperature dependence of the nanocrystal size is investigated in the range 200–1100¡ãC. The formation of a series of continuous solid solutions belonging to the tetragonal crystal system is revealed, and the limits of their thermal stability are determined.