The morphology of small Pd clusters on SiO2, TiO2, and Al2O3 and the influence of CO and the reaction mixture CO + O-2 on their fine structure have been investigated by TEM and EXAFS. For all supports, Pd particles have a quite narrow size distribution with mean diameters in the range 15-25 Angstrom, whereas their morphology and spatial homogeneity of location on the support are different. EXAFS data acquired in controlled-atmosphere cells have shown distortion of Pd surface layers to depend upon the nature of the support and/or the adsorbate. Disordering of Pd surface structure suggests appearance of the defect centers of CO and oxygen adsorption.