Intensification of red emission and improvement of crystallinity in SrTiO3:Pr3+ by Al addition and their correlation with Sr/Ti molar ratio have been studied. Rietveld analysis predicts that only a small amount of Al3+ can be incorporated into the Ti4+ site. Without Al, excess Sr in nonstoichiometric compositions is accommodated by precipitation of Ruddlesden-Popper phases, which consist of a SrO planar fault interleaved with SrTiO3 slabs. The planar faults are scavenged by added Al, as a result of precipitation of SrAl2O4. The improved crystallinity by the scavenging is one of the origins of the emission enhancement from SrTiO3:Pr3+.