The rates of many bimolecular reactions involving triplet excitons are magnetic field dependent at room temperature and for modest field strengths. The specific interactions discussed are the fusion of a pair of triplet excitons to yield either a single or triplet exciton, the interaction between a triplet exciton and a trapped triplet, the fission of a singlet exciton into a pair of triplets and the triplet excitonfree radical interaction. The magnetic field effects result from the influence

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... the field on the spin wavefunctions of the triplet exciton together with the existence of spin selection rules for the interactions. A simple theory of these effects is presented. Application of the theory to the experimental field dependence results yields new information about the mechanisms of these interactions.