The changes in activity and intermediate formation by the addition of zirconium to a Co/Mn/Br and Co/Ce/Br catalyzed aerobic p-xylene oxidation as a function of catalyst concentration, Co/Mn ratio, and Zr concentration are presented. There is a maximum activity increase at an optimum Co/Mn/Br concentration. As the degree of activation caused by Zr increases so does the ratio of the rate of benzaldehyde/rate of benzyl alcohol formation. While the most active Co/Mn/Br catalyst is at a 5/1 Co/Mn ratio, the most active Co/Mn/Zr/Br catalyst is at a 1/1 ratio. The coordination chemistry in acetic acid/water systems is summarized. The experimental data is rationalized by postulating that the benzyl hydroperoxide is dehydrated in the coordination sphere of the metals to give benzaldehyde and water, that the strong Lewis acidity of Zr(IV) is responsible for the changes in activity and selectivity, and that it occurs through a polynuclear Co/Mn/Zr coordination compound. (C) 2003 Elsevier B.V. All rights reserved.