This invention is directed to a process for removing a catalyst inhibitor from a substrate or catalytic converter containing at least one nitrogen oxide (NOx) catalyst using a phosphoric acid composition. The process is particularly suited for removing the catalyst inhibitor, arsenic, from a fly ash-coated substrate or converter. A substantial amount of catalyst inhibitor can be removed from a fly ash-coated catalytic converter without removing a significant portion of one or more of the NOx reduction catalysts in or on the substrate.