Hole burning as well as fluorescence line narrowing experiments have been performed on the system dimethyl-s-tetrazine in polyvinyl carbazole films at low temperatures. The first singlet electronic absorption bands are typical (300 cm-3 wide) of inhomogeneously broadened bands of guest molecules in amorphous organic hosts. Evidence is presented for both photochemical and nonphotochemical hole burning. The narrowest holes observed were lorentzian, had a width of 0.44 cm-1 at 1.8 K, and are believed to be of nonphotochemical origin. A model which envisions the guest molecules to occupy different sites in the polymer host with a distribution of energy barriers between sites is used to describe these observations. The fast (20 ps) relaxation time implied by the 0.44 cm-1 lorentzian linewidth is interpreted as indicative of the rate of site interconversion in the excited state. © 1981.