Conditions leading to cerium phosphate poisoning of both model and fully formulated automotive exhaust catalysts have been investigated. Temperatures above 700degreesC favor the formation of cerium orthophosphate (CePO4) as do rich engine operating conditions (i.e. excess fuel). No apparent differences were observed in the type or amount of cerium phosphate produced by impregnating ceria with Pt, Pd, or Rh. Cerium phosphate is produced both by reaction of ceria with gaseous H3PO4 (P2O5), and by reaction of ceria with other phosphorus compounds deposited on the catalyst (e.g. aluminum and zinc phosphates). These results provide the first direct evidence of a decrease in oxygen storage capacity associated with the irreversible conversion of CeO2 to CePO4, thus identifying CePO4 formation as a high-temperature mode of catalyst deactivation. (C) 2004 Published by Elsevier B.V.