The compositional range established here for gadolinium aluminate garnet is extraordinary. At its extreme, the composition Gd3.6Al4.4O12 (Gd/Al = 0.82) is closer to the adjacent perovskite phase GdAlO3 (Gd/Al = 1) than it is to its parent garnet phase Gd3Al5O12 (Gd/Al = 0.6). Rare-earth aluminates have long been considered to behave as line phases, and major departures from this behaviour (e.g. x > 0.03) have only recently been demonstrated in the analogous YAG system (stable up to x = 0.4) . In the related rare-earth gallate garnets RE3+xGa5-xO12, nonstoichiometries of a similar magnitude have very recently been reported with x < 0.2 (for RE = Gd 3+ ) and x < 0.6 (for the smallest rare-earth members Er, Yb and Lu) by conventional ceramic synthesis, but nonstoichiometric aluminates could not be made by this synthesis route: these are summarized in the Supporting Information (S38 -39). By a simple ionic radius argument, the Gd3+xAl5-xO12 (GAG) solid solution is expected to be less amenable than YAG or Gd3+xGa5-xO12 (GGG) to this type of nonstoichiometry. YAG and GGG have ionic radius ratios rA/rB of 1.68 and 1.51 respectively (using ionic radii of 6-coordinated Y 3+ , Gd 3+ and Ga 3+ ), but GAG has the greatest size mis-match between A and B cations with rGd/rAl = 1.75. Indeed, compared to YAG, the nonstoichiometric GAG system exhibits a more pronounced volume expansion with increasing x because Gd 3+ is larger than Y 3+ , as shown in Figure . The location of the compositional limit x = 0.6 at the half-way point between the ideal stoichiometries of garnet and perovskite is probably not coincidental, and may reflect strongly increasing competition from the crystallization of other stable phases which are closer in composition (notably ABO3-type phases). It is notable that a similar limit (x ≈ 0.5) is reported in the highly nonstoichiometric small-rareearth gallate garnets . In contrast, nonstoichiometry in the Y3+xAl5-xO12 system appears to be limited by the extent of the Y2O3 -Al2O3 glass-forming domain (at x = 0.4), rather than the proximity of competing phases. This also suggests that the local atomic arrangements in the precursor glasses (composed mainly of AlO4 and AlO5 species at x = 0) have an important role in phase selection, offering a more facile route to the crystallization of garnets (where at least 60% of Al 3+ is in tetrahedral coordination) than to perovskites (where Al 3+ is exclusively in octahedral coordination). Together, these factors should work to define the compositional limits for different RE 3+ across the 4f series: for each RE 3+ , the accessible nonstoichiometric range should depend on the preparation of a suitable activated precursor of the desired composition (glass, or other types such as a solution-derived precursor), up to an intrinsic limit of x ≈ 0.6. In addition to the interest of such materials for optical properties (discussed below), this could also open a new range of compounds of interest for their low temperature magnetic or magnetocaloric properties. The measurement of the PL spectra of several nonstoichiometric GAG phosphor types, along with previous study of the nonstoichiometric YAG system, allows a general picture to emerge about the luminescence responses of this materials class. It is the case in both GAG and YAG that upconversion phosphors are most strongly affected by increasing nonstoichiometry. In both examples, these phosphors follow complex emission schemes (two-photon absorption), contain small rare-earth activators (Er 3+ or Tm 3+ ) that populate two cation sublattices in the host material, and a high concentration of small rare-earth sensitizer (Yb 3+ ) with a similar crystallographic distribution to the activators. The nonstoichiometric YAG-based upconversion phosphors were previously studied to a limit of x = 0.2 and their spectra showed an approximately linear evolution with x. Here, the behaviour of (Gd0.89Tm0.01Yb0.1)3+xAl5-xO12 over a more extended range 0 ≤ x ≤ 0.4, indicates that this evolution will not necessarily remain linear as x increases to very high values, possibly due to different local configurations that can arise at such compositions. For example, the collapse in intensity of the 780 nm emission line between x = 0.2 and 0.4 (Figure ) coincides with a crossover-point where the number of Al 3+ ions removed from the B sublattice becomes larger than the number of dopant (Tb 3+ , Yb ) ions (see Supporting Information S29), permitting configurations with the optically-active species accommodated exclusively at the B sublattice. In-depth studies at low temperature may help to unravel the physical origin of the spectral changes observed here, for example a relaxation of selection rules to favor emission from electric dipole transitions (such as 1 G4-4 H5) over magnetic dipole transitions (such as 3 H4-3 H6). The exploration of other RE3+xAl5-xO12 systems may help to clarify such links between crystal chemistry and luminescence properties, and may be expected to yield a significant number of new modified upconversion phosphors.