The nitrogen formation efficiencies, defined as the number of nitrogen molecules formed for every reductant molecule consumed were determined for acetaldehyde and propionaldehyde in the reduction of NO2 under lean conditions over anatase TiO2. The efficiency increased with increasing NO2/reductant in the feed, reaching 0.61 +/- 0.05 for NO2/acetaldehyde between 1.5 and 3.0 and 0.85 +/- 0.08 for NO2/propionaldehyde between 3.8 and 5.7. Simultaneously, the CO/CO2 ratio as well as the small concentration of N2O in the product stream increased. The results suggested that at higher NO2/reductant ratios, the reaction between the reductant and adsorbed NO2 accounted for a large majority of the reaction. The results were consistent with the IR results, which showed that surface nitro groups reacted readily with acetaldehyde. The implications of these results on the NO2 reduction mechanism were discussed.