Samples extracted from a vehicle-aged commercial TWC monolith (the inlet part of the first cartridge) have been shown to contain CePO4. The effects of calcination temperature in the 773-1173 K range on these samples have been explored aiming at studying the stability against oxidation of the detected CePO4. The recovery of the Oxygen Storage Capacity (OSC) and therefore of the catalytic properties of the aged TWC by means of decomposition of CePO4 has also been explored; the CePO4 is suspected to play a role in the chemical deactivation of TWC through OSC damaging. XRD studies evidenced that CePO4 was very stable against calcination even at the high temperature of 1173 K. X-ray photoelectron spectroscopy (XPS) analysis confirmed the stability of the Ce(III) present in the aged sample. Tests with a simulated exhaust gas composition showed that the calcination procedures applied did not regenerate the catalytic activity of the aged TWC. In addition, temperature-programmed reduction (TPR) studies in H-2/He flow and CO/O-2 pulses have shown that the OSC of the aged sample was not recovered either. Although other effects can be responsible for TWC deactivation, the results of the present work support the view that CePO4 remains one of the reasons for the deactivation of TWC. It is proposed that the deactivation occurs through the formation of patches or islands of this solid phase around the Ce-Zr-O mixed oxide and CeO2 crystal particles, thus reducing significantly the rates of oxygen storage and back-spillover processes occurring through the participation of these oxides and the precious metal surface. (C) 2003 Elsevier B.V. All rights reserved.