In the present paper the influence of Mn₂O₃ was studied, as additive to sintering of CeO₂, in the absence or presence of calcium oxide. For these samples the relative density has values above 90% of the theoretical. The phase composition determined by X-ray diffraction, showed that manganese introduced by MnCO₃, determines at firing a complex oxide-reduction phenomena that lead to successive Mn₂O₃, Mn₃O₄ and MnO₂ formation. For the sample with a more amount of calcium oxide a binary compound Ca₂MnO₄ was identified.