Supported chromium oxide systems are widely used catalysts. Experimental data indicate that modification of the silica support with Al changes the properties of the surface Cr species. In this computational work, extensive density functional theory investigations of the monomeric Cr(VI) oxo species variously located on the Al-modified silica surface have been done using a large number of systematically generated periodic and cluster models. It is predicted that the surface modification can influence both the geometry and the relative stabilities of the Cr species. Monooxo Cr(VI) species located in the vicinity of two Al sites usually adopts a pseudotetrahedral geometry, in contrast to square pyramidal monooxo Cr(VI) species on silica. Under dehydrated conditions, the relative stabilities of the tetrahedral monooxo and dioxo Cr(VI) species are comparable, whereas the dioxo Cr(VI) species is thermodynamically preferred on silica. The obtained results explain the experimentally estimated higher ratio of the monooxo to dioxo surface Cr(VI) species for CrO3/AlOx/SiO2, compared to the chromia–silica system. Calculated Cr═O stretching frequencies for the monooxo and dioxo Cr(VI) species on the Al-modified silica surface are in a range similar to those predicted for the corresponding Cr(VI) species on silica, in agreement with reported experimental data.