Ce3+ luminescence was studied in the system Y1.98Ce0.02Si3-xAlxO3+xN4-x with a weighed-out x of 0, 0.25, 0.5, 0.6, and I. The lattice parameters show a nearly linear increase between x = 0 and x = 0.6, indicating an increasing substitution of Si-N by the larger Al-O in the melilite-type lattice. The: solubility limit is near x = 0.6; for higher values of x, the lattice parameters remain constant. The luminescence spectra show typical Ce3+ luminescence (excitation maxima at 310 and 390 nm and emission maximum near 475 nm). No shifts in the excitation spectrum and only slight shifts in the emission for increasing x were observed, indicating that a change in the overall composition does not affect the local coordination of the Ce3+ ion. This is explained by the preferential occupation of the large Ce3+ ion on "roomier" O-rich sites, as compared with the average coordination around Y3+ in Y2Si3O3N4. As a result of the preferential Ce3+ coordination, extra O2-, introduced with the incorporation of Al-O in Y2Si3-xAlxO3+xN4-x, will substitute on N-richer sites, which preferentially coordinate with the smaller Y3+ ion. The minimal shift of the emission spectrum results in a slightly larger Stokes shift (from about 4200 to 4300 cm(-1)), which suggests a decreasing rigidity of the host-lattice for increasing substitution of Si-N by Al-O. This is explained by the preferential substitution of Al on Si sites near Ce3+, which counterbalances the deficit in negative charge due to extra oxygen versus nitrogen in the Ce3+ coordination.