To explore the response of other classes of phosphor to nonstoichiometry, we prepared two representative systems showing (i) broad-band emission by Ce 3+ doping (Gd0.98Ce0.02)3+xAl5-xO12 and (ii) narrow-band emission by Tb 3+ doping (Gd0.9Tb0.1)3+xAl5-xO12. In contrast with the Tm/Yb upconversion system, and due to the difficulties in contrasting Gd 3+ and other RE 3+ ions by different techniques (see Section 3.4), the luminescence responses of these phosphors are presented here without precise experimentally-determined information on the dopant distribution within the crystal structures (although atomic scale STEM-EDS data do suggest an unquantified amount of Tb 3+ at the BO6 sites in (Gd0.9Tb0.1)3.4Al4.6O12 -see Supporting Information S35-37). The emission spectra and luminescence lifetimes of the Ce 3+ and Tb 3+ phosphors are shown together in Figure , and their structural and compositional analyses are detailed in Supporting Information sections S31-34 and S35-37 respectively. In (Gd0.98Ce0.02)3+xAl5-xO12, the main emission line centered at ~600 nm undergoes a slight broadening with an associated red-shift as nonstoichiometry (x) increases. At the same time, its luminescence lifetimes decrease, with average decay times of 49, 35 and 31 ns for x = 0, 0.2 and 0.4 respectively. These trends are very similar to those exhibited by nonstoichiometric YAG:Ce 3+ , where Ce 3+ populates the AO8 sublattice only, and the line broadening is ascribed to increasing disorder in the second coordination sphere of Ce 3+ . In contrast, the PL spectra and lifetimes of the narrow-band (Gd0.9Tb0.1)3+xAl5-xO12 phosphors are essentially insensitive to nonstoichiometry, representing typical behavior for this class of activator.