A new approach to prepare nanoparticles in SrF₂-YbF₃ systems via the fluorolytic sol–gel synthesis is presented. A series of Sr_1−xYb_xF_2+x (0.12 < x < 0.62) nanoparticles of nonstoichiometric fluorite phases obeying Vegard's law throughout the homogeneity area have been successfully prepared at room temperature. For all Yb concentrations XRD patterns show the formation of cubic phases. For higher Yb concentration (>50 mol%) crystallite sizes decrease abruptly in comparison to lower Yb concentrations (<50 mol%) as detected by XRD. The obtained sols are highly transparent. The investigations by DLS and TEM revealed the presence of monodisperse sol particles with sizes in the lower nm range (∼5 nm). The nanoparticles are characterized by a wide range of homogeneity (up to approx. 60 mol% Yb) and show promise of a wide range of applicability in the areas of medicine, dentistry and optics.