The low temperature singlet-triplet absorption line shapes of single crystals of naphthalene and 1,4-dibromonaphthalene are used to investigate the dynamic properties of triplet excitons. The line shapes although asymmetric, can be related to the approximate lifetimes of the k ≈ 0 exciton states. The temperature dependence of the line half-widths can be explained by acoustic phonon or lattice defect scattering at temperatures below 20°K. For 1,4-dibromonaphthalene an optical phonon mode begins to contribute to the line broadening above 20°K. The experimental linewidths indicate that exciton transport passes from an incoherent to a coherent process as the temperature decreases from 30 to 4°K. © 1973, The American Institute of Physics. All rights reserved.