Recharging processes of chromium ions were investigated for Mg 2 SiO 4 :Mg, Cr single crystals using annealing in O 2 and in air and γ-irradiation, as compare to YAG :Ca, Cr single crystals. The formation of tetravalent Cr ions in the Mg 2 SiO 4 :Mg, Cr is related not only to the initial Cr content in the melt, oxygen partial pressure and O 2-vacancy existing in the crystal, but also to the external field such as γ-irradiation. The additional absorption after γ-irradiation shows the decrease in intensity of the absorption of Cr 3+ and Cr 4+ ions in some part of the spectrum and increase in the other giving evidence on recharging effects between Cr 3+ and Cr
4+. There arises also color centers observed between 380 nm and 570 nm that may participate in energy transfer of any excitation to Cr 4+ giving rise to Cr 4+ emission. Opposite to forsterite crystal, absorption spectrum of YAG:Ca, Cr crystal after γ-irradiation reveals only increase in the absorption of the Cr bands. The observed behavior of the absorption spectrum of YAG:Ca, Cr crystal under influence of γ-irradiation suggests that γ-irradiation ionizes only Cr ions.