The zero-phonon R-line luminescence spectra of 3d3ions (Cr3+and Mn4+) in a series of Y3Al5–5yGa5yO12(0 ≤y≤1) solid solution garnet ceramics were studied. Instead of increased inhomogeneous line broadening which is usually observed in solid solutions, discrete structure in the Cr3+emission spectra is observed at lower values of y, whereas at high values of y the strong inhomogeneous broadening of the R-lines occurs and no discrete disorder response is observed. The results are explained based on the limited number of possible high-symmetry luminescent center geometries and the non-random preferential occupancy of tetrahedral lattice sites with gallium ions.