Using a novel method for analyzing transient photoluminescence (PL) experiments, a microscopic description is obtained for the dye concentration dependence of triplet-triplet annihilation (TTA) in phosphorescent host-guest systems. It is demonstrated that the TTA-mechanism, which could be a single-step dominated process or a diffusion-mediated multi-step process, can be deduced for any given dye concentration from a recently proposed PL intensity analysis. A comparison with the results of kinetic Monte Carlo simulations provides the TTA-Forster radius and shows that the TTA enhancement due to triplet diffusion can be well described in a microscopic manner assuming Forster- or Dexter-type energy transfer. (C) 2016 Elsevier B.V. All rights reserved.