The valence states of Cr ions in Ca or Ca/Mg co-doped Cr:Y3Al5O12 (YAG) single-crystal fibers are studied. The fibers were grown using the laser-heated pedestal growth method, followed by annealing treatments up to 1500 degrees C. The concentrations of the Cr3+ and Cr4+ ions in octahedral and tetrahedral sites in oxygen or nitrogen environments were characterized. Above 700 degrees C, migration of Cr4+ between octahedral and tetrahedral sites takes place; its relative stabilization energy was estimated. For Ca,Cr:YAG annealed in an oxygen or nitrogen environment, it was 0.25 and 0.3 eV, respectively. For Mg,Ca,Cr:YAG annealed in oxygen or nitrogen, it was 0.47 and 0.49 eV, respectively. For the Ca,Cr:YAG crystal fiber (Ca/Cr = 113.1%) with oxygen annealing, about 35% and 2.5% of Ca. ions took part in charge compensation for Cr4+ in the octahedral and tetrahedral sites, respectively. The density of oxygen vacancies depends on the concentration of Ca ions. The estimated ratios of the unreacted oxygen vacancies to total oxygen vacancies were about 63% and 88% for oxygen and nitrogen annealing, respectively. The main limitation on the concentration of Cr4+ in the tetrahedral site of YAG is the presence of unreacted oxygen vacancies. (C) 2008 Elsevier B.V. All rights reserved.