Here, we investigate the use of a tin(IV) sulfide complex (Sn2S6 4-) as a low-vibrationalenergy inorganic ligand for NaYF4 UCNPs (Scheme 1). We explored chemical approaches to replace the native oleate ligands with the Sn2S6 4-ligands and confirmed the successful ligand exchange via Raman and energy-dispersive X-ray spectroscopy (EDS). Optically, we found that the UCL efficiencies of UCNPs were enhanced by this exchange, with the greatest enhancements observed for small and highly doped UCNPs (showing UCL enhancements up to a factor of 16).