The controlled calcination of the chromium containing polyhedral oligosilsesquioxane (c-C₅H₉)₇Si₇O₉- (OSiMe₃)O₂CrO₂, 3b, monitored by MS analysis of the evolved gases and thermogravimetry measurements, led to the formation of a microporous Cr–Si–O amorphous mixed oxide containing 10.2 wt.% of chromium. The textural properties of the material as well as the speciation and dispersion of the chromium oxide on the silica surface were investigated by the use of nitrogen physical adsorption, XRD, XPS as well as DRS, RS and IR techniques. The nitrogen physical adsorption indicates a high surface area, a rather large pore volume and a very narrow pore size distribution around 6 Å diameter. The spectroscopic analysis of the material suggests a rather high dispersion of the chromium oxide species on the silica surface mainly as monochromate and to a certain extent as dichromate and clusters of Cr₂O₃. The material was briefly tested in the ammonia oxidation reaction and was found to have a catalytic activity, which was slightly higher than that of a chromium oxide on a silica reference catalyst.