Raman and emission spectroscopy was used for the characterization of dysprosium–yttrium, holmium–yttrium, and ytterbium–yttrium mixed oxides. The samples, which are potential emitter materials for thermophotovoltaic generators, were prepared by combustion synthesis. Due to the similar ionic radii of the trivalent rare earth cations employed, substitutional solid solutions are formed. With changing yttrium content, continuous changes in Raman band position and linewidth occur for the mixed oxides, which were analyzed for selected vibrational modes. By this method, it is possible to determine the composition of the samples. In addition, emission spectroscopy was used for the characterization of the holmium–yttrium mixed oxides. A correlation between the holmium concentration in the yttrium oxide matrix and spectral features was established. By using confocal emission microscopy, local variations in emission band profiles, which may be related to variations in chemical composition, were investigated with micrometer resolution.