Here we propose to measure the phonon density of states in nanocrystalline samples of YAG:Cr. Our parallel luminscence measurements have revealed dramatic changes in the phonon assisted luminescence side-band peaks as well as size induced changes in decay times in nonradiative (phonon-driven) relaxation spectra of this system. The energies of interest that couple to the optically active ions are ~12meV, and in this preliminary proposal, we plan to explore the size-induced changes in the PDOS of one such concentration YAG:Cr(1%). The entire phonon bandwidth will be explored.