Reaction of aqueous AgNO3 with aqueous M-3[Cr(ox)(3)] in greater than or equal to3:1 molar ratio causes the rapid growth of large, cherry-black, light-stable crystals which are not Ag-3[Cr(ox)(3)], but [M-0.5(H2O)(3)]@[Ag2.5Cr(ox)(3)] (ox(2-) = oxalate, C2O42-; M = Na, K, Cs, Ag, or mixtures of Ag and a group 1 element). The structure of these crystals contains an invariant channeled framework, with composition {[Ag2.5Cr(ox)(3)](-0.5)}(infinity), constructed with [Cr(ox)(3)] coordination units linked by Ag atoms through centrosymmetric {Cr-O2C2O2-Ag}(2) double bridges. The framework composition [Ag2.5Cr(ox)(3)](-0.5) occurs because one Ag is located on a 2-fold axis. Within the channels there is a well-defined and ordered set of six water molecules, strongly hydrogen bonded to each other and some of the oxalate 0 atoms. This invariant channel plus water structure accommodates group 1 cations, and/or Ag cations, in different locations and in variable proportions, but always coordinated by channel water and some oxalate 0 atoms. The general formulation of these crystals is therefore [MxAg(0.5-x)(H2O)(3)]@[Ag2.5Cr(ox)(3)]. Five different crystals with this structure are reported, with ;compositions 1 Ag-0.5[Ag2.5Cr(ox)(3)](H2O)(3), 2 Cs-0.19 Ag-0.31[Ag2.5Cr(ox)(3)](H2O)(3), 3 K0.28Ag0.22[Ag2.5Cr(ox)(3)](H2O)(3), 4 Cs-0.41,Ag-0.09[Ag2.5Cr(ox)(3)](H2O)(3), and 5 Cs0.43Ag0.07 [Ag2.5Cr(ox)(3)](H2O)(3). All crystallize in space group C2/c, with a 18.4, b approximate to 14.6, c approximate to 12.3 Angstrom, beta approximate to 113degrees. Pure Ag-3[Cr(ox)(3)](H2O)(6) which has the same crystal structure (1), was obtained from water by treating Li-3[Cr(ox)(3)] With excess AgNO3. Complete dehydration of all of these compounds occurs between 30 and 100 degreesC, with loss of diffraction, but rehydration by exposure to H2O(g) at ambient temperature leads to recovery of the original diffraction pattern. In single crystals, this reversible dehydration-hydration occurs without visually evident crystal change, but with loss of mechanical strength. We postulate a general mechanism for transport of water molecules along the channels, associated with local partial collapses of the channel framework, with concomitant bending but little breaking of the host Ag-O and Cr-O bonds, which is readily reversed.