Novel red emitting KCaBO3:Eu phosphors have been synthesized by solid-state reaction at various temperatures. Systematic studies on annealing effects and consequent structural evolution and optical properties were investigated by various structural and photoluminescence studies. With an increase in annealing temperature (from 700 °C to 950 °C), these phosphors show a gradual change from a mixed low crystalline phase to a highly crystalline single phase, with minimized volatile impurities. Photoluminescence studies revealed that the low-temperature annealed phosphors showed distinct mixed emission composed of blue and red emissions upon UV excitation. Such dual emission is due to the coexistence of Eu3+ and Eu2+ ions, wherein the reduction of Eu3+ into Eu2+ was attributed to the presence of volatile impurities. Relatively high-temperature annealed phosphors exhibit strong red color photoluminescence due to homogeneously occupied Eu3+ ions in the host crystal charge-compensated (with K+ ions) sites of Ca2+ ions. The dominant red-to-orange emission intensity ratios and Judd-Ofelt parameters of Eu3+ ions support the strong covalent nature and site-occupation of higher asymmetry sites of K+ and Ca2+ ions. High emission life times and efficient and stable photoluminescence at different excitation wavelengths make these phosphors suitable for white LEDs and other display applications. © The Royal Society of Chemistry 2012.