A near-UV excited, oxyfluoride phosphor solid solution Sr 1.975 Ce 0.025 Ba(AlO 4 F) 1Àx (SiO 5 ) x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce 3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (l em ¼ 523 nm) to yellow (l em ¼ 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 AE 5% for some of the examples in the series. Excellent thermal properties were found for the x ¼ 0.5 sample, with the photoluminescence intensity at 160 C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (l max ¼ 400 nm) exhibit high color rendering white light with R a ¼ 70 and a correlated color temperature near 7000 K. The value of R a could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.