The contaminants present in a real three way catalysts (TWC) aged under real working conditions for ca. 30 000 km have been determined. The X-ray fluorescence (TXRF) and energy-dispersive X-ray analysis (EDS) showed that P, Ca, Zn, Pb, Cr, Ni, Fe, Cd and Cu are present in the used catalysts. Distribution within the washcoat was scrutinised by scanning electron microscopy (SEM-EDS) analysis. The more external region (10–15 μm) of the washcoat are very concentrated in Ca and P. CePO4 was detected by X-ray diffraction (XRD). Other phases that can be formed between contaminants themselves and with the washcoat components were not detected by XRD. Temperature-programmed reduction (TPR) experiments show that an important fraction of the Ce present in the washcoat is unable to participate in Ce3+/Ce4+ redox pair under oxidising–reducing treatments. CePO4 is proposed to explain the locking of the Ce3+/Ce4+ redox couple. Although other contaminants like Pb can be also poisoning the catalyst, the formation of CePO4 can be invoked to explain the worse catalytic properties displayed by used sample. The locked Ce pair is unable of participating in the rapid Ce3+/Ce4+ redox couple required for correct oxygen storage capacity (OSC) properties and for the proper functioning of TWC.