A high-nuclearity polyoxopalladate compound, [Pd-2(II)subset of{(H7Pd15O10)-O-II(PO4)(10)}](9-) {Pd-17(II)}, comprising a {Pd-15} host occupied by a {Pd-2} guest and the parent pristine "empty" [(H7Pd15O10)-O-II((PO4)-O-V)(10)](13-) {Pd-15} cluster have both been prepared and characterized by single-crystal X-ray crystallography, P-31 NMR, CSI-MS, and XPS. The encapsulated {Pd-2} has a short Pd-II-Pd-II distance within the {Pd-15} host. Solution studies indicate that the empty host and filled guest complex are in equilibrium with each other, and UV titrations revealed a binding constant of ca. 10(3) for the guest Pd-II ions, with a binding stoichiometry of almost 2.