Abstract The 2E → 4A 2 emission spectra of Cr:NaMgAl(C 2O 4) 3−9H 2O have been recorded as a function of Cr 3+ concentration in the range 0.1 to 100%. The spectral changes in the R lines are ascribed to species in multiple sites and not to exchange interactions. Energy migration to quenching sites occurs at high concentrations via the 4T 2 (or 2T 1) state. Energy transfer in the 2E state is inefficient at all Cr 3+ levels. The vibronic sideband intensity is enhanced relative to the R lines as the Cr 3+ concentration is increased.