CoO-MgO samples (CoxMg1-xO:x = mol of Co / (mel of Co + mol of Mg)) were prepared by impregnation of MgO powder with an aqueous solution of Co(NO3)(2), followed by calcination at 773 K. The atomic concentration range of Co for formation of a solid solution and the state of Co ions dissolving in the samples were investigated mainly by XAFS. XRD patterns showed that the dominant phase is MgO in the samples of x < 0.3 and the formation of small Co3O4 crystallite occurs in the samples of x > 0.3. For the samples of x > 0.5, the dominant phase was Co3O4 and the MgO phase was not detected by XRD. Co K-edge XANES spectra demonstrated that the octahedral symmetry around Co ions in the sample of x = 0.02 is distorted in the samples of x greater than or equal to 0.04, indicating the change in the state of dissolved Co ions. Co K-edge EXAFS spectra revealed that Co ions dissolve into MgO matrix substitutionally in the samples of x < 0.04 while the phase separation due to the formation of the Co3O4 structure occurs in the samples of x greater than or equal to 0.04.