Fluorescence of naphthalene (N) labeled to poly(methyl metracrylate) (PMMA) particles sterically stabilized by polyisobutylene (PIE) was studied by transient fluorescence method in iso-octane, hexadecane, and in powder form. When samples of dispersions are annealed above 60 degrees C, their room temperature lifetimes increase, and when powder samples are so annealed, lifetimes decrease. Self-quenching effects among N*/N pairs were eliminated to isolate unimolecular lifetimes. Low-frequency intramolecular vibrations are found responsible for the temperature dependence of nonradiative transition of excited N molecules. Corresponding activation energies are measured and found to be between 2300 and 1100 cal/mol. Solvent effects on these activation energies are discussed.