We show that a fundamental assumption used in analyzing photon echo decays from interacting molecules - the assumption that the "pure" dephasing rate can be obtained by subtracting one half of the fluorescence decay rate (2T1)-1 from the photon echo decay rate-is generally inconsistent with a fully quantum mechanical treatment. The density matrix evolution for a system of noninteracting dimers is calculated using the retarded electromagnetic coupling between two oscillating dipoles. We are able to properly include the effects of spontaneous emission and Dicke superradiance on the echo decay. © 1988 American Institute of Physics.