In this study, optical spectroscopy, excited state dynamics and the Tb³⁺→Eu³⁺ energy transfer in particular have been investigated in detail (both from the theoretical and experimental point of view) in eulytite double phosphate hosts A₃Tb(PO₄)₃ (A=Sr, Ba) doped with Eu³⁺. It has been found that the energy transfer is strongly assisted by fast migration in the donor Tb³⁺ subset. Moreover, the transfer rates and efficiencies depend significantly on the nature of the divalent elements present in the structure and hence on the distances between nearest neighboring Tb³⁺ and Eu³⁺. It is shown that the competition between quadrupole-quadrupole and exchange interaction is crucial in accounting for the transfer rates.