This report attempts to further define the underlying principles governing activation of the Phillips Cr/silica catalyst. Calcining the catalyst causes dehydroxylation or annealing of the silica surface that generates the active site population, and thereby the catalyst activity and polymer character. The extent of dehydroxylation during activation depends not only on the calcining temperature, but also on the calcining time, making the two variables seemingly inter-convertible in their effect on catalyst and polymer behavior. However, the rate of surface annealing also depends on temperature, so that the degree of time dependence can vary with the choice of activation temperature. These facts can be used to explain commercial behavior and to design more efficient activation recipes. (c) 2007 Elsevier B.V. All rights reserved.