A hitherto unknown type of aqueous complex, ternary Ca-M-IV-OH complexes (M = Zr and Th), causes unexpectedly high solubilities of zirconium(IV) and thorium(IV) hydrous oxides in alkaline CaCl2 solutions (pH(c) = 10-12, [CaCl2] > 0.05 mol.L-1, and pH(c) = 11-12, [CaCl2] > 0.5 mol.L-1, respectively). The dominant aqueous species are identified as Ca-3[Zr(OH)(6)](4+) and Ca-4[Th(OH)(8)](4+) and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The number of OH- ligands in the first coordination sphere detected by EXAFS, N-O = 6 (6.6 +/- 1.2) for Zr and N-O = 8 (8.6 +/- 1.2) for Th, are consistent with the observed slopes of 2 and 4 in the solubility curves log [M](tot) vs pH(c). The presence of polynuclear hydrolysis species and the formation of chloride complexes can be excluded. EXAFS spectra clearly show a second coordination shell of calcium ions. The [Zr(OH)(6)](2-) and [Th(OH)(8)](4-) complexes with an unusually large number of OH- ligands are stabilized by the formation of associates or ion pairs with Ca2+ ions. The number of neighboring Ca2+ ions around the [Zr(OH)(6)](2-) and [Th(OH)(8)](4-) units is determined to be N-Ca = 3 (2.7 +/- 0.6) at a distance of RZr-Ca = 3.38 +/- 0.02 A and N-Ca = 4 (3.8 +/- 0.5) at a distance of RTh-Ca = 3.98 +/- 0.02 A. The Ca-3[Zr(OH)(6)](4+) and Ca-4[Th(OH)(8)](4+) complexes have first (M-O) and second (M-Ca) coordination spheres with the Ca2+ ions bound to coordination polyhedra edges.