Electronic absorption, excitation and time-resolved emission spectra of Tb3+ in the cubic elpasolite host lattice Cs2NaYF6 show that the D-5(3)-D-5(4) cross-relaxation is due to an electric quadrupole-electric dipole mechanism. The donor Tb3+ ion transits from D-5(3) to D-5(4) whilst the acceptor ion transits from F-7(6) to F-7(2) with the emission of an odd-parity phonon. (c) 2011 Elsevier B.V. All rights reserved.