Crystallographic and physical properties of cubic Er2−xNi21B6, Yb2−xNi21B6 and Lu2−xNi21B6 have been studied. Their crystal structures have been established from single-crystal and powder X-ray diffraction data (space group Fm3¯m, Cr23C6 type of structure): Er2−xNi21B6, a = 10.6520(2) Å, RF = 0.041, RW = 0.044 (for x = 0.18); Yb2−xNi21B6, a = 10.6412(2) Å, RF = 0.021, RW = 0.022 (for x = 0.14); a = 10.6243(2) Å, RF = 0.023, RW = 0.025 (for x = 0.30); Lu2−xNi21B6, a = 10.6269(2) Å, RF = 0.023, RW = 0.023 (for x = 0.35). Small homogeneity ranges extend from x = 0.1–0.9 for Er2−xNi21B6 and from x = 0.1–0.3 for Yb2−xNi21B6. No homogeneity range was found for Lu2−xNi21B6 (x = 0.35). Magnetization measurements for Yb2−xNi21B6 and Er2−xNi21B6 reveal the 4f13 (Yb3+) and 4f11 (Er3+) electronic state and no magnetic ordering above 1.8 K. RE2−xNi21B6 compounds show metallic behavior in electrical conductivity.