Phosphorescence spectra and lifetimes are presented for 1,2,3,4-, 1,3,5,7-, 1,4,5,8-, and 2,3,6,7-tetrachloronaphthalene in EPA. The unusual results obtained for 1,4,5,8-tetrachloronaphthalene are rationalized on the basis of steric interactions between adjacent chlorine atoms. Attempts to find a simple relationship between the magnitude of the internal heavy-atom effect and the positions and number of heavy atoms are also briefly discussed.