There is not yet a straightforward answer concerning the mechanism(s) of selective catalytic reduction of NO,, by hydrocarbons. In this study, a systematic approach of this reaction was undertaken over 0.5 wt.% Pd/Al2O3. Successively, oxygen, NO and C3H6, which is the reductant, were either suppressed or substituted by NO2, acetone, propanal, 1- or 2-nitropropane. A ''memory effect'', i.e. N-2 formation in (NO + O-2) after (NO + O-2 + C3H6) reaction, was observed. Moreover, a concurrence exists between the overshoot in the CO2 formation and N-2 formation peaks in (NO + O-2 + C3H6) reaction under transient conditions, continuously raising the temperature. The more the reactants were adsorbed, the higher the nitrogen production amount was. To interpret these results, strong adsorption phenomena have to be invoked. From the results obtained with the use of carbonyl and nitro-compounds, it seems that both species may be involved in the DeNO(x) reaction mechanisms.