At high brightness, triplet-triplet annihilation (TTA) reduces the efficiency of organic light-emitting diodes. Triplet diffusion may considerably enhance this effect, which is otherwise limited by the rate of long-range interactions. Although its role can be clarified by studying the emissive dye concentration dependence of the TTA loss, we demonstrate here the practical applicability of a more direct method, requiring a study for only a single dye concentration. The method uses transient photoluminescence yield measurements, for a wide initial excitation density range. The analysis is applied to an iridium complex and is supported by the results of kinetic Monte Carlo simulations. (C) 2016 Elsevier B.V. All rights reserved.