The nature of reduced Cr species on silica is still under debate. In this work, combined DFT and in situ UV-vis DRS investigations of reduction of chromia-silica catalyst have been done. Advanced periodic and cluster models of amorphous silica are applied to reproduce the heterogeneity of surface Cr species. Geometrical parameters, vibrational frequencies, excitation energies, and relative stabilities of Cr sites are determined. It is predicted that reduction of monomeric Cr(VI) species to Cr(IV) sites is highly exergonic, whereas deep reduction to Cr(II) is less thermodynamically favored. Dimeric Cr(VI) species can finally be reduced to Cr(III) or Cr(II). In situ UV-vis DRS studies indicate that Cr(VI) species are selectively converted to Cr(II) under CO in dry conditions, whereas reduction with dry H-2 leads to formation of Cr(III) and Cr(II). For both reducing agents, the presence of water hampers deep reduction to Cr(II), which can be explained by reoxidation/hydrolysis processes. (C) 2016 Elsevier Inc. All rights reserved.