AbstractThis paper reviews the kinds of long‐lived photoluminescence that can be observed in fluid solutions, and summarises the applications of the measurement of such photoluminescence. These applications include the measurement of triplet lifetimes and triplet radiative lifetimes, triplet energy, triplet formation efficiency, triplet quenching rates, probability of triplet‐to‐triplet energy transfer; the observation of excited dimers and excited complexes; the determination of trace impurities and the products of photochemical reaction. By working in perfluorocarbon solvents the method can be applied to reactive triplets such as the π*‐ntriplets of aromatic