Oxidation of propene, but-l-ene or acetylene in O-2 sub-stoichiometry was studied over two alumina-supported catalysts (1 and 3% Pd) and over on unsupported palladium oxide. Structural changes of palladium in these reactions was investigated by XRD, TEM and electron diffraction as well as by TG-DTG analysis. Palladium, initially in the form of PdO, reduces to a new cubic phase denoted PdX with a lattice parameter (0.398 nm) higher than in metallic palladium obtained by reduction in H-2 (0.389 nm). PdX can be observed by XRD at the very beginning of propene oxidation, which is confirmed by electron diffraction patterns taken on several Pd particles after reaction. This new phase of palladium is specifically formed in alkene and alkyne oxidation, which contrasts with what was: observed in propane oxidation during which PdO reduced into Pd. TG-DTG analysis and XRD results obtained at various O-2/propene ratios support the hypothesis that PdX is formed by O insertion into the cubic structure of metallic palladium. (C) 2001 Elsevier Science B.V. All rights reserved.