Reported are two isostructural Zintl compounds, CaNa10Sn12 and SrNa10Sn12, with mixed alkali and alkaline-earth cations and isolated clusters Of Sn-12(12-) with the shape of giant truncated tetrahedra. The compounds were synthesized by heating the corresponding mixtures of elements at 950 degreesC. The structures were solved and refined from single-crystal X-ray diffraction data in the cubic space group I (4) over bar 3m (No. 217), where Z = 2 with a = 11.1847(6) and 11.2176(4) Angstrom for CaNa10Sn12 and SrNa10Sn12, respectively. Both compounds are diamagnetic and therefore electronically balanced.