The Energy and Environmental Research Center installed a portable slipstream selective catalytic reduction (SCR) reactor in the convective pass of a utility boiler cofiring wood waste and Powder River Basin (PRB) coal. Catalyst sections were removed after 43, 128, and 171 days of operation. SCR catalytic activity was determined for each section, and a sample of one section was examined under a scanning electron microscope to look for signs of catalyst blinding and/or poisoning. The catalyst deactivated at an apparently inverse rate with an initial deactivation rate of 18% every 1000 h. The major mode of deactivation appeared to be pore blocking by combined alkali and calcium sulfate deposition and growth. Published by Elsevier Ltd.