This report attempts to define the underlying principles governing activation of the Phillips Cr/silica catalyst during fluid bed calcining. Cr(VI) is destabilized at high temperatures by water vapor, and therefore the key to a successful activation is in balancing two conflicting goals: (1) achieving extreme dehydroxylation of the silica surface, while (2) simultaneously minimizing Cr(VI) exposure to the H2O by-product. Moisture can lead to the growth of large crystallites of alpha-chromia which are difficult to reoxidize. Three main variables can be used to control moisture concentration in the gas stream during commercial activation-catalyst charge weight, air flow, and thermal ramp profile. In this study the sensitivity of Cr(VI)/silica to moisture, or to the absence of O-2, has been defined as a function of temperature. Such laboratory data then allows one to model and predict the performance of Cr/silica catalyst activations when carried out on a commercial scale using a wide range of recipe variables. (c) 2007 Elsevier B.V All rights reserved.