In order to study the effects of spin transitions on the equation of state of Mg-perovskite, we performed in situ synchrotron X ray powder diffraction on Fe³⁺-bearing Mg-perovskite using NaCl or Ne pressure medium. The sample was annealed in measurements using Ne pressure medium, whereas it was not annealed in the runs using NaCl pressure medium. The unit cell volumes of runs using Ne pressure medium are smaller than those using NaCl pressure medium. The difference in compression curves may be caused by the difference in stress conditions and/or Fe²⁺/Fe³⁺ ratios of the samples during the experiments.