Looking to the past, heading to the future. In this contribution we explain the reasons why the Cr/Al2O3 Phillips catalysts exhibit a faster kinetics profile in ethylene polymerization reaction with respect to Cr/SiO2. Diffuse reflectance UV-Vis and FT-IR spectroscopies unequivocally demonstrate that, albeit several types of reduced Cr sites are stabilized by the Al2O3 support, only the 4-fold coordinated Cr2+ sites are active precursors in ethylene polymerization, as for Cr2+/SiO2. Nevertheless, kinetic experiments indicate that ethylene polymerization is 15 times faster on CO-reduced Cr/Al2O3 than on CO-reduced Cr/SiO2. The difference is even more striking (two order of magnitude) when the reaction rates per active Cr sites are compared. Our experimental results suggest two reasons behind the faster polymerization kinetic of Cr/Al2O3: (1) the higher ionic character of the Cr-O-Al bond with respect to the Cr-O-Si one; (2) the nature of the ancillary ligands in the coordination sphere of the Cr active sites (which are mainly carbonates for CO-reduced Cr/Al2O3 and siloxane bridges for CO-reduced Cr/SiO2). (C) 2017 Elsevier Inc. All rights reserved.