Abstract: The interaction between γ-Al_2O_3 and the oxides of Zinc, Cadium and Mercury have been studied by x-ray diffraetion(XRD). Samples of ZnO/γ-Al_2O_3, CdO/γ-Al_2O_3 and HgO/γ-Al_2O_3 were prepared by impregnating γ-Al_2O_3 with the corresponding nitrate solutions, then drying and ealeinating. It has been shown that CdO and HgO can be dispersed on γ-Al_2O_3 surface to form submonolayers. Their dispersion capaeieies derived from Fig.2 and Fig.3 are 0.48g CdO/g γ-Al_2O_3 and 0.16g HgO/g γ-Al_2O_3, or 0.20g CdO/100m~2 and 0.067g HgO/100m~2 surface of γ-Al_2O_3 (specific surface of γ-Al_2O_3 is 240m~2/g). On the other hand, ZnO/γ-Al_2O_3 system can not be in a monolayer dispersion state, because ZnO can react with γ-Al_2O_3 to form Zinc spinel at rather low temperature (200 ℃). The difference between these three systems may be attributed to their different ion radius.