We have synthesized [Pt(CN-iso-C3H7)4][M(CN)4] (M = Pt, Pd) and studied their reversible hydration and sorption properties with UV-vis, FT- IR spectroscopy, and X-ray diffraction. Powder diffraction studies show that anhydrous [Pt(CN-iso-C3H7)4][Pt(CN)4] and [Pt(CN-iso-C3H7)4][Pd(CN)4] crystallize in a tetragonal space group with nearly identical lattice constants. Gravimetric studies reveal that variable guest-host stoichiometries occur when solid [Pt(CN-iso-C3H7)4][Pt(CN)4] sorbs the guest at room temperature from the gas phase [water, 12.1 (1) molecules per formula unit, chloroform 6.0(1), methanol 8.0(1), and trifluoroethanol 4.1(1)]; these sorption processes are reversible. The unit cell distances in the tetragonal ab-plane expand dramatically when the solvent guests are sorbed, but changes along the c-axis (the M-M direction) are minimal. Crystallization of [Pt(CN-iso-C3H7)4][Pt(CN)4] from water gives monoclinic crystals of a hexadecahydrate [Pt(CN-iso-C3H7)4][Pt(CN)4] · 16H2O. This salt consists of alternating cation/anion chains along b with an average Pt-Pt distance of b/2 = 3.1521(1) Å. The sixteen water molecules per formula weight interlace neighboring chains via H-bonding with each other and the CN- ions of the Pt(CN)42- units. The shifts in the UV-vis and IR spectra that occur when solvent guests are sorbed by the double complex salts are discussed in terms of the lattice expansions that are observed. A mechanism for the lattice expansions that accompany the sorption of guest molecules is proposed.