The evolution dynamics of Cr4+, Cr3+ and Cr2+ contents in chromium doped forsterite (Cr:Mg(2)SiO4) laser crystals during their prolonged high-temperature multi-stage oxidizing annealing have been investigated for the samples grown by Czochralski in 'standard' slightly oxidizing atmosphere (Ar + 2,2 vol% O-2), as well as in inert atmosphere (argon, the residual O-2 content is 0,01 vol%). The contents of Cr4+ increase by factor of 1,5-3 during the first 800-1000 h of the annealing and then do not change substantially. Cr2+ practically disappears from the crystals within 800-900 h of the annealing. The content of Cr3+ passes through a maximum during the annealing. Therefore, this content is determined by two parallel reactions: Cr2+. Cr3+ and Cr3+. Cr4+.