Abstract Y2O3, Lu2O3, and Sc2O3 are isomorphous, crystallizing in the cubic space group Ia3, and Gd3+ can readily be incorporated into each of these three compounds. The rare earths occupy two different sites with symmetry C2 and S6. As a consequence, the Gd 3+ 6 P 7 2 → 8 S transition is split into eight crystalfield lines. Other electronic transitions as well as vibronic side bands are also observed and the differences between the spectra of the three compounds are discussed.