In many cases energy transfer occurs between the ions1–8and therefore its efficiency depends not only on the diffusion of ions but on the Coulomb interaction. Diffusion causes ions to draw nearer. Consequently the rate of energy transfer increases because this rate decreases with increasing distance between the sensitizer (S) and the activator (A). The presence of the Coulomb interaction leads to the redistribution of ions A near the ion S. As a function of a sign of ionic charge the concentration of A near the ion S will be more or less in comparison with the random concentration of these ions in solution. Since the Debye radius of shielding can be compared with the critical radius of energy transfer (Förster's radius) the Coulomb interaction between ions often causes considerable changes in the observed efficiency of energy transfer.