This paper focuses on the kinetics of Cr⁴⁺ formation in Cr,Ca:YAG ceramics prepared by solid-state reaction sintering. The kinetics of Cr⁴⁺ formation was studied by annealing of Cr,Ca:YAG ceramics in ambient air under different temperatures at different times, resulting in the transformation of Cr³⁺ to Cr⁴⁺. The activation energy (E_a) of Cr³⁺ oxidation determined by the Jander model was 2.7 ± 0.2 eV, which is in good correlation with the activation energy of innergrain oxygen diffusion in the YAG lattice. It is concluded that Cr³⁺ to Cr⁴⁺ transformation in YAG ceramics is limited by oxygen diffusion through the grain body. It was established that in Cr,Ca:YAG ceramics, the intralattice cation exchange, in which the Cr⁴⁺ ions exchange positions with the Al³⁺ ions, switching from “A” to “D” sites, is faster than Cr³⁺ to Cr⁴⁺ oxidation. In the temperature range of 900–1300 °C, the reaction enthalpy of Al³⁺/Cr⁴⁺ ion exchange between octahedral “A” and tetrahedral “D” lattice sites is close to zero, and this exchange ratio is thermodynamically driven by entropy.