The adsorption of acetaldehyde and its coadsorption and reaction with NO, were investigated on a Na-Y,FAU zeolite using in situ FTIR spectroscopy. Acetaldehyde adsorbs strongly over Na-Y and desorbs molecularly at around 400 K with very limited extent of condensation or polymerization. Reaction between CH3CHO and NO, takes place in coadsorption experiments even at 300 K. In the initial step, acetaldehyde is oxidized to acetic acid accompanied by the formation of NO, which can be observed as N2O3 formed via a further reaction between NO and NO2. The key intermediates in the overall NOx reduction in this process are nitromethane and, possibly, nitrosomethane, which form in the next step. Their decomposition and further reaction with adsorbed NOx species lead to the formation of HCN, HNCO, N2O, CO2, and organic nitrile species identified by their characteristic IR vibrational signatures. At 473 K, the reaction between adsorbed CH3CHO and NO, is very fast. The results seem to suggest a mechanism in which N-N bond formation takes place among ionic nitrogen containing species (NO+ and CN- or NCO-). No evidence has been found to suggest the participation of NHx+NOy- type species in the N-N bond formation under the experimental conditions of this study, although their role in the overall N-2 formation process cannot be ruled out under realistic catalytic conditions.