Energy transfer from UO 22+ to Sm 3+ is described. The transfer efficiencies are calculated from the decrease of donor luminescence and lifetimes and from the increase of the acceptor fluorescence. It is shown that the transfer is nonradiative. The energy transfer efficiencies are greater when the donor is excited at higher energy levels due to stronger overlap between electronic levels of donor UO 22+ and acceptor Sm 3+. From the comparison of energy transfer efficiencies from UO 22+ to Sm 3+ and Eu 3+ it is deduced that the overlap between excitation levels of donor and acceptor is a sufficient condition for the transfer.