A series of acid-treated zirconia-pillared montmorillonites doped with carbon and manganese was studied in selective catalytic reduction (SCR) of NO with NH3. For selected catalysts, various stability tests were performed. Catalysts M2C and M2CMn14 showed very good stability in long-run tests with the use of a standard reaction mixture. Addition of water vapour caused small decrease in NO conversion but the effect was reversible. SO2 poisoning gave different results depending on the applied method. In the conditions well-simulating DENOX process, i.e, when SO2 was added in the course of catalytic reaction, the activity of the studied catalysts did not change.