In situ X-ray absorption near-edge spectroscopy/extended X-ray absorption fine structure (XANES/EXAFS) experiments are conducted. for the first time on a highly diluted Cr/SiO2/Si(100) system (2 Cr/nm(2), representing a model of the Phillips catalyst for the ethylene polymerization) by exploiting the reflection EXAFS (ReflEXAFS) geometry. This experiment, aimed to give a contribution in bridging the gap between surface science and catalysis, demonstrates that it is possible to follow the reversible red-ox reactivity of surface species grafted on a single well-defined surface, at a concentration limit that is far below the monolayer coverage level and for a highly sensitive sample. A further improvement on the impurity level of the ReflEXAFS chamber is however required in order to be able to follow in situ the polymerization reaction. Our results demonstrate that the red-ox ability of the isolated surface Cr species is not enough to make a polymerization active species.