Cr-doped mesoporous silica glasses with Cr loading in the 0.01-0.5 wt% range were synthesized by one-pot acid catalyzed sol-gel route in form of monoliths. The absence of light scattering in the whole IR and UV-Vis-NIR region makes them ideal systems to investigate the optical and structural properties of grafted Cr sites by means of transmission spectroscopy, even in the highest diluted case, on which this contribution is mainly focused. For these reasons, Cr-doped porous silica monoliths are proposed here as models for the well-known Phillips catalyst for ethylene polymerization. It is demonstrated that, even when chromium is extremely diluted (0.01 wt%), the electronic and structural properties of the Cr sites (both in the oxidized and reduced forms) are very similar to those of Cr sites on standard, aerosil-based, Phillips catalysts (with a Cr content of 0.5 and 1 wt%): a distribution of Cr sites differing in the coordination ability is always present, irrespective of the Cr concentration and of the silica support. Nevertheless, the possibility to perform UV-Vis spectroscopy in transmission allowed to add unprecedented information on the relative intensity of bands due to charge transfer and d-d transitions. In situ ethylene polymerization was followed by means of both IR and UV-Vis spectroscopy in transmission on the sample having the lowest Cr loading (0.01 wt%), showing that the precursor species can be easily traced from the first steps of polymerization and that they are very similar to those observed on more concentrated catalysts. (C) 2013 Elsevier Inc. All rights reserved.