Abstract: The systems of CrO_3/SiO_2 and CrO_3/γ-Al_2O_3 have been studied by X-ray diffraction (XRD). The CrO_3/SiO_2 (or CrO_3/γ-Al_2O_3) samples with various amounts of CrO_3 were prepared by mixing CrO_3 and SiO_2 (or γ-Al_2O_3) in dry atmosphere, and by calcining at temperatures below the melting point of CrO_3 (196 ℃) for 24 hrs. In order to protect the samples from moisture in the XRD measurement, a special sample holder (see Fig.1) was used. The maximum dispersion amounts (dispersion threshold)of CrO_3 on SiO_2 at various temperatures (101 to 170 ℃) have been obtained from the XRD quantitative phase analysis (see Fig.3). It has been shown that the dispersion thresholds change with calcination temperatures. For CrO_3/SiO_2 system, as temperature changes from 101 to 170 ℃, the threshold increases from 0.27 to 0.38 g CrO_3/g SiO_2 (see Fig.4). For CrO_3/γ-Al_2O_3 system, as temperature changes from 120 to 171 ℃, the threshold changes from 0.22 to 0.42 g CrO_3/g γ-Al_2O_3 (see Fig.5). The specific surfaces of the SiO_2 and γ-Al_2O_3 are CrO_3 on SiO_2 and for CrO_3 on γ-Al_2O_3 at 150 ℃ are 0.096 and 0.191 g CrO_3/100 m~2 respectively. According to a closepacked monolayer model, the monolayer capacity of CrO_3 on a support should be 0.081 g CrO_3/100 m~2. The experimental results indicate that CrO_3 dispersed on SiO_2 or γ-Al_2O_3 are not monolayer. They are quite different from those systems we reported before. In thoe systems the active components could disperse spontaneously onto the carrier surface to form a monolayer or submonolayer and have definitive dispersion thresholds. The dispersion thresholds of CrO_3 on SiO_2 or γ-Al_2O_3 increase with calcination temperature, and can exceed their monolayer capacities. These behaviours can be attributed to the polymerization of CrO_3. We suggested that heating at a lower temperature CrO_3 reacted with SiO_2 to form surface compound and heating at a higher temperature CrO_3 formed homopoly-acid-radical.