We introduce a novel, phosphorescent, colloidal silica sphere tagged with eosin-5-isothiocyanate as a tracer for rotational diffusion on the millisecond time scale measured in situ with time-resolved phosphorescence depolarization. The suitability of the tracer was investigated by a combination of spectral and time-resolved fluorescence and phosphorescence measurements. The phosphorescence quantum yield of dye molecules is high, because they are effectively shielded for quenching by oxygen or solute molecules, though some oxygen and water may penetrate the silica sphere. Both energy transfer between eosin molecules and internal rotational mobility of eosin cause a limited decay of the initial phosphorescence anisotropy. Finally,the hydrodynamic radius of the tracer found from the time-resolved phosphorescence anisotropy experiment coincides with the value found by dynamic light scattering experiments. Our results show that silica tracer spheres tagged with eosin are suitable tracers for rotational diffusion measurements.