XRD and surface area determinations have been carried out on samples of a commercial ceria-baria-alumina on cordierite monolith heated in air or H2/He at temperatures from 500 to 1200°C. Below 700°C the fall in surface area is slightly greater in the reducing atmosphere but stability is better above 1000°C. Sintering in air proceeds with continuous growth of CeO2 particles and the development of α-Al2O3 above 1000°C. Traces of a previously unreported phase are evident after 24 h above 1100°C. CeO2 is barely detectable after treatment in H2/He and less α-Al2O3 is formed. Instead the predominant crystalline phase (apart from cordierite) is CeAlO3 at temperatures to 1100°C while the new phase forms at 1200°C in considerably greater amounts than under oxidising conditions. Sintering for different periods of time at 1200°C shows that the new phase arises with consumption of α-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 approx. 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.