In both GAG and YAG the Ce 3+ phosphors are only weakly perturbed by nonstoichiometry, associated with Ce 3+ retaining an 8-fold first-coordination sphere (shown experimentally for nonstoichiometric YAG:Ce 3+ ). The PL response of Ce 3+ is highly sensitive to its local coordination environment, and the incorporation of significant concentrations of Ce 3+ at the BO6 sublattice might be expected to produce strong changes in emission properties. Strategies to achieve this may include development of highly nonstoichiometric Eu3+xAl5-xO12:Ce 3+ hosts. In such a system, the smaller ionic size contrast between Eu 3+ and Ce 3+ may help to homogenize their distribution between A and B sites. Finally, whilst the indifference of the narrow-band emitter Tb 3+ to nonstoichiometry suggests that there is little interest in pursuing other highly nonstoichiometric phosphors of this type, there is scope for other types of nonstoichiometric phosphor (beyond those reported here) to be developed, for example in Gd3Al5O12 -Gd3Ga5O12 (GAG-GGG) solid solutions which are an important class of persistent luminescence host.