Radiationless energy transfer between rare-earth ions (Ln3+) in solutions has some features: 1) the electronic transitions in Ln3+complexes causing the luminescence of energy donor and the absorption of energy acceptors are forbidden by Laporte's rule and are weakly intensive. Therefore the critical radius (Ro) of energy transfer between the rare-earth ions for dipole-dipole mechanism is close to that for exchange-resonant mechanism. This fact presents difficulties for unequivocal interpretation of the energy transfer mechanism. 2) The plus-three lanthanide ions exist in solution as a set of complexes with different number of charged ligands in the inner coordination sphere and hence with different total charge of complexes.