La_2-xCe_xCuO₄ shows the usual Cu²⁺ EPR line. However, after use as catalyst for NO reduction by CO, the x = 0.2 sample shows also an unusual EPR feature, with apparent g values ranging between 5 and 2.5 depending on temperature. Cu⁴⁺, substituting for La³⁺, could transform into Ce³⁺ by removing one electron from the Cu-O₂ plane. However this, by itself, would not account for the unusual behaviour of the x = 0.2 used the sample, as a very small amount of Ce³⁺ substitutes for La³⁺ ions, nearly identical in all of the examined samples. We propose that undissolved CeO₂ partially transforms into Ce₂O₃ and acts as a sink of O^2-. The last ions would transform into O^- by reacting with the O atoms brought about by the NO reduction. When CeO₂ is enough concentrated, as for x = 0.2, O^- "spin bags" would form and interact with Cu²⁺ ions system.