We present a thorough investigation of nonradiative energy-transfer processes in various rare-earth (R) pentaphosphates (RP5O14). Using time-resolved fluorescence spectroscopy, different crystals with high and low concentration of the interacting R3+ ions were investigated. It turns out that energy transfer in RP5O14 causes both spatial energy migration of excited states and fluorescence quenching. At high rare-earth concentration the concentration dependence of fluorescence quenching is shown to be governed by fast energy migration. From low-concentration measurements the dominant interionic coupling mechanism could be determined employing a microscopic picture for the energy-transfer process. A particular statistical model is introduced to combine the results obtained in the low- and high-concentration limit. The investigations yield that energy transfer in RP5O14 is due to electric multipole interactions within the entire range of rare-earth concentrations, even at interionic spacings of 5. © 1981 The American Physical Society.