Co-SSZ-13 zeolite catalysts were prepared by solid-state ion exchange with CoCl2 salt for the selective catalytic reduction of NO with methane (CH4/NO-SCR). Fresh catalysts contained hard-to-reduce lattice Co2+/[Co-OH](+) ions and showed activity proportional to the concentration of these species. The NO conversion was accompanied by steam-generating methane oxidation. Under the arising hydrothermal conditions, a fraction of Co ions transformed into Co-oxo species especially in catalysts of high Co content (Co/Al-F > 0.3), resulting in boosted activity in NO2-forming catalytic NO oxidation. The increased NO2 concentration brought about higher concentration of surface cobalt nitrate species responsible for the rate-determining methane activation, so the overall rate of the NO-SCR reaction was also accelerated. Steam admixed to the feed decreased the conversion of methane more significantly than that of NO. Its observed high hydrothermal stability renders the Co-SSZ-13 catalyst a very promising candidate for the CH4/NO-SCR reaction. (C) 2020 The Author(s). Published by Elsevier Inc.