Highly efficient green-emitting (Sr,Ba)(3)Si6O3N8:Eu phosphors are prepared by solid-state reaction. Their optical properties are adjusted to be suitable for white light-emitting diode (LED) application. It is found that there are two ways to efficiently tune the excitation and emission spectra of this material system. One method is to adjust the ratio of Sr2+ ions to Ba2+ ions in the host lattice. The other method is to control the europium concentration. The luminescent intensity of the phosphor sample is optimized at Sr2.8Ba0.2Si6O3N8:Eu-0.2. In this case, a synergy effect in the photoluminescence intensity is observed when substituting Ba2+ for Sr2+ ions while keeping the Sr3Si6O3N8 crystal structure as the dominant phase. The excitation spectrum of the phosphor sample is broad due to the 4f-5d transitions, which is favorable to InGaN chip application. Also, an emission wavelength is easily tuned to the peak wavelength of 538 nm with the full width of half-emission maximums of 77 nm. Temperature-dependent photoluminescence intensity exhibits a low thermal quenching. We believe that this green oxynitride phosphor can be used as a solid-state lighting source. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3533687] All rights reserved.