Semiempirical Interacting Bonds Method was used to calculate the enthalpies of oxygen removal from the bulk of ceria and ceria-zirconia solid solutions. The energy of the bulk reduction was obtained to agree reasonably with the experimental data when the complex defect comprised of the oxygen vacancy and interstitial oxygen atom surrounded by six cations was suggested to emerge due to an oxygen removal from the bulk. The zirconium cations dissolved in ceria affect the energies of the bulk reduction and bulk oxygen mobility due to a decrease in the local Me-O coordination number from 8 to 7.