The reaction of nitroethane with NO in the presence of O2 has been investigated over Cu-MFI. For short periods on-line, conversion is complete above 250°C with CO2 and N2 as the products. However, at temperatures below 330°C accumulation of deposits leads to deactivation with the appearance of isocyanates and CO. FTIR gas analysis confirms the formation of toxic methyl isocyanate (CH3NCO), with yields approaching 25% along with smaller amounts of isocyanic acid (HNCO). Nitromethane reacts similarly but deactivation to give HNCO alone is an order of magnitude slower. Emergence of HNCO is accompanied by significant amounts of HCN and NH3. Nitroethane reacts in O2 alone in a manner similar to that in NO/O2 but with less than half the initial N2 yield. Deactivation is faster and small amounts of HCN and NH3 appear concurrently with CH3NCO and HNCO. In the nitroethane system, HNCO, HCN, and NH3 arise from decomposition of deposited material as demonstrated by the temperature-programmed decomposition in helium. The main reaction pathway with the two nitrocompounds appears to be rearrangement followed by dehydration to the corresponding isocyanate and hydrolysis to amine (or ammonia), with the latter steps proceeding on Brønsted acid sites. Nitrogen is then formed by reaction with NO/O2 on the transition metal as in the ammonia-selective catalytic reaction which is fast over Cu-MFI. A parallel route to N2 involves reaction of NO2 with deposited material but this can occur only as long as the catalyst retains activity for the oxidation of NO. Otherwise deactivation ensues with appearance of isocyanate. Selective catalytic reduction of NO using ethane over Cu-MFI in excess O2 yields some NO2 in which case the various reactions involving nitroethane would be sufficiently fast for it to be an intermediate in this system.