Table 2 summarizes some qualitative features of this kind of long-range transfer and of some more or less trivial mechanisms. The non-trivial transfer differs from re-absorption transfer by its independence of the volume of the solution, by the decrease in sensitizer fluorescence lifetime, and by the invariability of the sensitizer fluorescence spectrum. It differs from short-distance collisional transfer by its independence of solvent viscosity and from transfer within a molecular complex by the constancy of absorption spectra and the decrease in sensitizer fluorescence lifetime. In most cases, some of these different properties allow a decision between trivial and non-trivial transfer mechanisms. Further discriminations may be made by quantitative studies of these properties.