We synthesized a single-phase MgAlSiN3:Eu2+ phosphor, which exhibited a tunable red emission, by controlling Eu2+ concentration. MgAlSiN3:Eu2+ phosphors that have a low Eu2+ concentration and emit red light could be suitable for use in white light emitting diodes, and MgAlSiN3:Eu2+ phosphors with a higher Eu2+ concentration that emit a deep red light at around 710 nm could be an energy down-converting material for use in some other applications. Time-resolved photoluminescence was also examined as a function of Eu2+ concentration and suggests an energy transfer between Eu2+ activators from different local structures. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3525608] All rights reserved.