It is shown that a nature of the LFER phenomenon is related to phase volumes of the system's states, for which a difference of free energy is determined. An invariability of these volumes is the single factor generating the LFER phenomenon. As an example, the LFER manifestations for an intramolecular excimer formation with 1,3-di(lpyrenyl)propane in various solvents are analyzed. (c) 2006 Elsevier B.V. All rights reserved.