Abstract: We present X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) investigations of ordered chromium oxide ultrathin films prepared on a Pd(111) single-crystal surface. The films where grown by thermal evaporation of Cr under an oxygen atmosphere and sample temperature of 600 K. The ordered films produced are strongly dependent on the film thickness and annealing treatment. Films with thickness below 5 A?, produced at low coverages, display a p(2 x 2) structure relative to Pd(111). Thicker films (thickness>10 A?) always have a ( ?3 p x ?3 p) R30° structure. The photoemission measurements were done using conventional X-ray sources as well as synchrotron radiation taken at the Laboratório Nacional de Luz Síncrotron. The XPD data were interpreted through the use of a multiple scattering calculation approach combined with a genetic algorithm for surface structure optimization. Combining the information from XPS, low energy electron diffraction and XPD measurements we have determined the surface structure of the ( ?3 p x ?3 p) R30° phase. Elsewhere, we report on the structure of the p(2 x 2) phase Ver menos