The saturation coverage behavior of Cr(VI) on silica has been used to study the mobility of hexavalent chromium during commercial activation of the Phillips catalyst in a fluidized bed. Cr(VI) was found to migrate not only within each silica particle, but even between particles. The mechanism of transfer is thought to be particle-to-particle contact during the fluid bed calcining process, and Cr migration was observed at temperatures as low as 300-400 degrees C in these experiments. Transfer can take place even in the absence of oxygen, which suggests that redox cycling is probably not the main method of mobility. However, once the silica is calcined at high temperatures the migration is stopped at low temperatures, indicating that the thermal history of the surface is important in the transfer process. Migration of Cr on and between alumina particles behaved in much the same way, and Cr transfer was also observed between silica and alumina in both directions. Cr(III) on deactivated commercial Cr/silica catalysts could be reclaimed to Cr(VI) surprisingly easily by co-activation with virgin support. The spreading out of Cr(VI) onto a fresh unoccupied surface probably provides a powerful force to reoxidize the otherwise seemingly inert alpha-Cr2O3 crystallites. The mobility has important consequences for commercial manufacture and use of the catalyst. (C) 2007 Elsevier Inc. All rights reserved.