In this in situ EXAFS and XANES study on the Phillips ethylene-polymerization Cr/SiO2 catalyst, two polymerization routes are investigated and compared. The first mimics that adopted in industrial plants, where ethylene is dosed directly on the oxidized catalyst, while in the second the oxidized catalyst is first reduced by CO at 623 K. On this reduced catalyst C2H4 polymerization has been investigated at room temperature and at 373 K. To allow experiments in transmission mode, a Cr loading of 4 wt% has been adopted. At this loading a fraction of clustered Cr2O3 particles has been observed and quantified. The use of a third-generation synchrotron radiation source has allowed us to improve the energy resolution and signal-to-noise ratio of the XANES data, allowing us to determine the fraction of Cr sites involved in the polymerization reaction. This number represents an upper limit of the active sites. Preliminary ReflEXAFS experiments have been performed on a model catalyst prepared by impregnation of Cr on a flat Si(100) substrate covered by a thin layer of amorphous silica. Experiments have been performed ex situ on the grafted catalyst (i.e., after impregnation and thermal activation) and at the end of the polymerization stage.