Lanthanum-based perovskite-type oxides represented by LaBO₃ (B = Co, Fe, Mn) have been thought to present strong limitations for practical application although they are active for catalytic removal of NO. Cerium (Ce) substitution has been extensively studied to modify the properties of perovskites. It is noted that a new phase of ceria (CeO₂) can be separated from perovskites when the doping ratio exceeds the solution limit (x > S). This review outlines the relationship between the existence of CeO₂ phase and catalytic activity. CeO₂ dispersing on the lattice surface or small particles are beneficial for catalytic activity, but larger particles are adverse. Ce-doped LaBO₃ perovskites exhibiting the best activity must contain additional CeO₂ phases. In addition, CeO₂-supported LaBO₃ perovskite catalysts are discussed.