Surface modifications of bulk and supported palladium catalysts have been investigated between 25 and 300 °C under controlled atmospheres in the presence of 5×10−3atm NO and/or 5×10−3atm CO by means of in situ Raman spectroscopy. The evolution of the Raman spectra of bulk and alumina supported palladium under NO reflects the ability of Pd to dissociate adsorbed NO molecules. The subsequent accumulation of oxygen atoms leads to the ultimate formation of subsurface oxygen species and/or PdO islands. Successive NO and CO adsorptions on 1wt% Pd/Al2O3 lead to an oxidised or reduced palladium surface, respectively. On the other hand when CO and NO are simultaneously introduced, the competitive adsorptions between these two reactants, more in favour of NO, lead to an irreversible formation of PdO. These observations have been discussed in the light of mechanistic aspects obtained from previous kinetic investigations.