The mechanism of the reduction of the Phillips CrOx/SiO2 catalyst with ethylene is still not determined. In this work, full reaction pathways for this process, including further transformations of the oxygenated products, are calculated. The mechanism of the CrOx/SiO2 reduction with CO is also studied. It is predicted that the most kinetically favored reduction mechanism involves the reaction between ethylene and both oxo ligands of the surface dioxo Cr(VI) species, leading to formation of Cr(II) site and two formaldehyde molecules. We show that formaldehyde oxidation to carbon oxides and water over the dioxo Cr(VI) sites can be kinetically more accessible than conversion to methyl formate over Cr(II) sites. Formaldehyde can also easily generate various surface intermediates. The calculated activation barrier for the reduction of the CrOx/SiO2 system with CO is higher than in the case of the reduction with ethylene. (C) 2018 Elsevier Inc. All rights reserved.