Investigation of triplet excitation energy dynamics in naphthalenecholeic acid crystals and isotopically mixed naphthalenecholeic acid crystals are described. Phosphorescence, sensitized phosphorescence of beta-methylnaphthalene trap, and phosphorescence decay are observed at 16 and 4.2 K to characterize the energy-transfer dynamics. In neat stoichiometric crystals (33 mol % C10H8), the phosphorescence decays by unimolecular processes with a lifetime (2.7 +/- 0.1 s) characteristic of naphthalene. This exponential decay and the absence of delayed fluorescence indicate that exciton annihilation and impurity trapping do not occur in this system under the experimental conditions. In the isotopically mixed crystals, unimolecular decay dominates at low concentrations of C10H8 (5 mol %, 28 mol % C10D8) At higher concentrations of C10H8, energy transfer to P-methylnaphthalene traps occurs at early times (less than or equal to 300 ms) after excitation until the exciton density is too low relative to the trap density for transfer. At long times then, unimolecular processes again dominate. At a C10H8 concentration of 20 mol %, the energy-transfer rate in isotopically mixed naphthalene crystals is 35 times larger than in the naphthalenecholeic acid system. From agreement between theory and the experimental data, it appears that a one-dimensional exchange interaction topology is consistent with triplet excitation energy transfer in this system.