A physical model based on resonant electric dipole interaction between excited activators is presented that may be used for gaining insight into the second‐order energy‐loss process in cathode‐ray‐irradiated Tb phosphors. It is shown that resonant up‐conversion of the5DJexcitons (J=3,4) to highly excited 4fand 5dstates may be of equal importance. The derivation of the rate constants for this second‐order energy‐loss process is given and experimental values for the5D4‐5D4and5D3‐5D4interaction are presented for Tb‐doped YAG, YAGaG, LaOBr, and Y2SiO5. The values depend on previously determined optical cross sections for excited‐state absorption. Based on this model we explain well‐known differences in saturation behavior between these Tb‐doped phosphors under cathode‐ray excitation.