Single crystals of Eu-151:YAG were grown by the vertical Bridgman method using Eu2O3 with isotopic enrichment of 151Eu of 97.5%. Additional Hf(4+)or Si4+ ions were introduced to favor a high concentrations of Eu2+. As compared to Eu:YAG with natural isotope composition, the EPR spectra of YAG doped with 151Eu isotope show a reduced number of hyperfine structure components and a well-resolved structure of a bigger number of electronic transitions. Optical properties of obtained crystals and the effects of heat treatments under oxidizing and reducing conditions are reported. Based on the analysis of Eu3+ distribution in oxidized Eu,Hf:YAG, in comparison to that in Eu:YAG, the concentration of Eu2+ in as-grown Eu,Hf:YAG is determined. (C) 2016 Elsevier B.V. All rights reserved.