The magnetic anisotropy of RCo 1 0 Si 2 compounds has been studied in the temperature range from 4.2 to 300 K by means of AC-susceptibility and magnetization measurements. GdCo 1 0 Si 2 does not form and crystallizes in the 1:11 phase which has a very low Curie temperature compared with the 1:12 compounds. The anisotropy field of YCo 1 0 Si 2 decreases with increasing temperature. All 1:12 compounds investigated show easy-plane anisotropy at room temperature. In DyCo 1 0 Si 2 and HoCo 1 0 Si 2 , spin-reorientation transitions occur at 190 and 75 K, respectively, where the anisotropy changes to easy-axis type. In ErCo 1 0 Si 2 , at 70 K, the easy-magnetization direction changes from basal plane to cone.