Selective catalytic reduction (SCR) catalysts are deactivated by several mineral and metallic trace elements at highly variable rates determined by fuel quality and furnace firing conditions. With a loss in activity, NO is reduced over a longer inlet length of the SCR monolith, which leaves a shorter trailing section to sustain the most favorable conditions to oxidize Hg 0 and SO 2 . Since virtually no operating SCR was designed for Hg oxidation and since different monoliths are routinely combined as layers in particular units, the Hg oxidation performance of any SCR fleet is largely unmanaged. The analysis in this paper directly relates a measurement or manufacturer's forecast on the deterioration in NO reduction with age to corresponding estimates for oxidation of Hg 0 . It accommodates any number of catalyst layers with grossly different properties, including materials from different manufacturers and different ages. In this paper, the analysis is applied to 16 full-scale SCRs in the Southern Company fleet to demonstrate that catalyst deactivation disrupts even the most prominent connections among the Hg 0 oxidation performance of commercial SCRs and the behavior of fresh catalysts at lab, pilot, and even full scale.Implications: Catalyst deactivation confounds even the most prominent connections among the Hg 0 oxidation performance of commercial SCRs and the behavior of fresh catalyst at lab, pilot, and even full scale. The halogen dependence has been emphasized throughout the literature on catalytic Hg 0 oxidation, based on a large database on fresh catalysts. But for deactivated catalysts in commercial SCRs, the number of layers is much more indicative of the Hg 0 oxidation performance, in that SCRs with four layers perform better than those with three layers, and so on. The new qualified conclusion is that Hg 0 oxidation is greater for progressively greater HCl concentrations only among SCRs with the same number of layers, even for an assortment of catalyst design specifications and operating conditions. PAPER HISTORY