The dephasing of optically prepared k≈0 triplet excitons in crystalline (1,4)-dibromonaphthalene (DBN) has been investigated by measurement of the temperature dependence of the absorption line shape. At temperatures below 10 K the dephasing is due to scattering of the exciton by impurities or lattice defects. We show that the naturally occurring 13C containing DBN molecules can make a significant contribution to this linewidth. Above 10 K the excitons are scattered by optical phonons. The most likely exciton-phonon scattering process is shown to involve a single phonon scattering of the exciton between the two crystallographically inequivalent sites. Copyright © 1977 American Institute of Physics.