The phenomenon of triplet-triplet annihilation in the following mixed crystal systems is discussed: phenanthrene-h10in biphenyl, phenanthrene-d10in biphenyl, naphthalene-h8in biphenyl, naphthalene-d8in biphenyl and chrysene in biphenyl. The kinetics of the phenomenon are elaborated, particular attention being paid to tthe effects of (i) host-guest triplet energy gaps; (ii) random walk of triplet host excitons; and (iii) the influence of adventitious impurity or host crystalline defects. The topics discussed and concerning which experimental information is presented are: (a) the decay behavior of phosphorescence and delayed fluorescence; (b) the temperature dependence of the above decays; (c) the growth characteristics of phosphorescence and delayed fluorescence; (d) the temperature dependence of phosphorescence and delayed fluorescence intensities; (e) the concentration dependence of delayed fluorescence intensity; and (f) the incident intensity dependence of phosphorescence and delayed fluorescence intensities. It is shown that all data, with the single exception of delayed fluorescence decay at T ∼ T 77°K, are in accord with the kinetic model developed. The decay behavior of delayed fluorescence at 77°K is interpreted in terms of a crystal defect model. Exception is taken to the model of Hadley, Rast and Keller for the temperature dependence of phosphorescenc decay; it is shown that the annihilative model is in better accord with fact.