NO reduction activity with acetaldehyde over BaY is low in a dry feed, but significantly higher in the presence of water vapor. Aldol condensation of acetaldehyde takes place, polymerization of the resulting crotonaldehyde deactivates the catalyst. Water vapor prevents the formation of large amounts of carbonaceous deposits, either by suppressing the dehydration of aldol to crotonadehyde or by interacting with carbonaceous deposits or their precursors, transforming them to CO, CO2 and H-2. Temperature-programmed oxidation (TPO) shows much lower coke formation after tests in the presence of water vapor. TPD-MS data show that nitromethane, a possible intermediate in the NOx reduction process, reacts with water on BaY forming NH3, COx and isocyanic acid HNCO. NOx conversion over BaY reaches values near 90% at 200 degreesC and a GHSV of 30,000 h(-1) if the NOx initially contains both NO and NO2. No transition metal is involved in the reduction of NOx to NH3 and N-2 over BaY. (C) 2003 Elsevier B.V. All rights reserved.