XRD and surface area determinations have been carried out on samples of a commercial ceria-baria-alumina on cordierite monolith heated in air or H-2/He at temperatures from 500 to 1200 degrees C. Below 700 degrees C the fall in surface area is slightly greater in the reducing atmosphere but stability is better above 1000 degrees C. Sintering in air proceeds with continuous growth of CeO2 particles and the development of alpha-Al2O3 above 1000 degrees C. Traces of a previously unreported phase are evident after 24 h above 1100 degrees C. CeO2 is barely detectable after treatment in H-2/He and less alpha-Al2O3 is formed. Instead the predominant crystalline phase (apart from cordierite) is CeAlO3 at temperatures to 1000 degrees C while the new phase forms at 1200 degrees C in considerably greater amounts than under oxidising conditions. Sintering for different periods of time at 1200 degrees C shows that the new phase arises with consumption of alpha-Al2O3 and CeAlO3. It is fluorescent in ultraviolet light and can be separated as a heavy fraction by gravimetric settling. The XRD pattern closely resembles that of a known cerium terbium magnesium hexaaluminate phosphor and electron microprobe analysis suggests an approximate composition of Ce(Ba1-xMgx)Al11O19 (x similar to 0.87). The presence of magnesium shows that its formation involves mingling of cordierite and washcoat components in a process driven by the conversion of Ce4+ to Ce3+ in a reducing atmosphere. The new hexaaluminate:phase was observable by XRD in four out of 19 three-way converters recovered from used vehicles.