Energy transfer between different kinds of rare earth ions has been studied in various hosts coactivated by Ces and Tb3+17-14. It has been found that in all cases the excitation energy transfers from Ce

3+

to Tb

3+

. In investigating the. photoluminescence of YAG:Ce

Tb phosphors

however

excitation energy is found to transfer from Tb

3+

to Ce

3+

. This is shown in Fig.5 and 6 where a new excitation band appears at 270nm for YAG: Ce

Tb when the intensity of Ce

3+

emission at 520nm is monitored. This band excites Tb3+ luminescence quite efficiently in YAG:Tb

3+

(Fig.3) but is almost ineffective for the Ce

3+

520nm emission in YAGsCe

3+

(Fig.4)

unless the latter is codoped with Tb

3+

. These results imply that the energy absorbed by Tb

3+

ions transfers to Ce

3+

ions.