X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses have been used to characterize the structure of a La2O3-promoted MnOx/SiO2 catalyst, before and after its utilization in the oxidative dehydrogenation of ethylbenzene (EB). MnOx/SiO2 and MnOx/La2O3/SiO2 catalysts were prepared by pore volume impregnation, using aqueous solutions of (i) La3+- nitrateat an atomic ratio of La/Si = 0.08, and (ii) Mn2+-nitrate at an atomic ratio of Mn/Si = 0.10, followed by drying and calcination at 500 degrees C in air. XRD data show no diffraction patterns specific to MnOx on the La2O3-promoted MnOx/SiO2 catalyst, after calcination. Thus, the presence of La2O3 apparently favors the dispersion of manganese oxides during calcination, presumably by forming mixed Mn-La oxides. On the fresh promoted and unpromoted catalysts, after calcination, XRD and XPS analyses indicated that Mn was present mostly as MnO2 and Mn2O4. In the used catalyst, Mn from the unpromoted catalyst degenerated from Mn4+ to Mn2+, resulting in formation of Mn2O3 species, whereas in the case of La2O3-promoted catalyst Mn remained well dispersed as MnO2 and Mn2O3. It appears that La2O3 precludes the formation of Mn2O4 during the EB dehydrogenation, conserving Mn structure and oxidation state.