The microstructure of palladium particles deposited on a porous silica support was examined by high resolution microscopy following various sulfidation treatments. For catalysts sulfided by the reaction itself (hydrodesulfurization of thiophene), the micrographs show large particles where an amorphous sulfide layer surrounds the Pd core; in the case of small particles, the presence of PdS2 was identified. For catalysts sulfided under 20% H2S/H-2 at 623 K, the particles show lattice planes compatible with PdS2 and PdS. By increasing the sulfidation temperature to 723 K, faceted PdS particles are mostly formed. Some structural defects related to the sulfidation treatment are also shown.