Several technologies offer a promising strategy for the exhaust after-treatment of diesel engines in which continuing to further thrift Pt/Pd alloy diesel oxidation catalysts (DOC) by increasing Pd content, the prospect of an essentially Pt-free DOC catalyst is very attractive because Pd is cheaper than Pt. However, to determine the viability of this approach a much better understanding of PdO/AI20 3 chemistry and reactivity is needed. Therefore, the aim of this work is (1) to improve the catalytic performance of the Pd based ?iesel oxidation catalyst by using different preparation method and to investigate the effect of the thermal aging' on the catalyst activity towards CO and C3H6 oxidation. (2) clarify the effect of H20 and NO in the CO and C3H6 catalytic oxidation reactions in terms of reaction mechanism. (3) to investigate the catalyst deactivation induced by sulphur poisoning. It has been found that the structure, morphology and textures properties of the catalysts were significantly affected by both the preparation method and the thermal aging. In particular, the formation of highly catalytically active palladium nanoparticles showing a high density of defects and/or corners, have been observed when the sample was prepared by solution combustion synthesis and when thermal aging was carried out between 500 - 750 ·C. Regardless of the preparation method and the thermal aging, the presence of water in the reaction mixture resulted to promote the catalyst performances with the respect to CO oxidation which occurred via reaction with water derived species rather than dioxygen. Beside, water has also been found beneficial in modulating the NO, C3H6 and S02 inhibition. The formation of strongly bonded sulphate species on the catalyst surface along with the permanent loos of the structure and morphology properties of the catalyst, have been identified as the main cause of the catalyst deactivation induced by sulphur poisoning.