The three parameters, fluorescence polarization (P), relative quantum yield $(\phi)$, and lifetime $(\tau)$ have been reinvestigated for solutions of chlorophyll b in lecithin as a function of concentration. All three show different concentration dependence, of which the depolarization only, may be described by a simple single step energy transfer. It is proposed that the self-quenching occurs when nonfluorescent pairs of molecules are excited directly, and that the reduction in lifetime is a result of energy transfer through the bulk solution to such 'pairs' or other quencher. The origin of these 'pairs' of molecules may be shown to be purely statistical. The apparatus used for measurement of fluorescence lifetimes is described.