The site densities of the surface of different oxides and hydroxides have been calculated from crystallographic data. The following values for the common habits are recommended : 2.7 nm(-2) (Corundum), 2.3 nm(-2) (Boehmite and Gibbsite), 2.2 nm(-2) (Hematite and Rutile), 2.1 nm(-2) (Lepidocrokite), 1.9 nm(-2) (Diaspore), and 1.7 nm(-2) (Goethite). The probable deviations due to the variations in habits are about +/-0.2 nm(-2), except Rutile (+/-0.4 nm(-2)). For the concrete faces the values of site density are adduced.