The functionality of solid supramolecular metalo-organic materials stems from the noncovalent interactions from which they assemble, endowing them with dynamic properties that can be tuned and controlled. Incorporation of quaternary pyridinium salts (QPS) as building blocks brings into focus the self-assembly of donor-acceptor charge-transfer complexes (CTCs) with distinctive hydrochromic, thermochromic and piezochromic properties1. These optical properties are enhanced when external stimuli cause such structural donor-acceptor reorganization through delicate adjustment of valence and conduction bands that allow charge-separation via one-electron donor-to-acceptor transfer. Pyridinium oximes as QPS are recently recognized as promising electron-accepting and H-bond donating units for the design of 2D- and 3D-self-assemblies with hexacyanoferrate(II) as an electron donor and H-bond acceptor2,3. Here we present the structural and spectroscopic solid-state study of the stimuli-responsive CTC between N-benzylpyridinium oxime (BPA4+) and hexacyanoferrate(II) (HCF4–)4. The in-situ PXRD was used to investigate temperature-induced dehydration of the parent, reddish-brown (BPA4)4HCF·10H2O (1·10H2O) to anhydrous blue (BPA4)4HCF (1) providing evidence of a fully reversible hydrochromic behavior. The crystal structures of both phases are resolved by SCXRD2,3 reveling the extent of structural donor-acceptor reorganization upon dehydration (Fig. 1). The crystal packing of 1, where the hydroxyl group within oxime moiety forms a direct H-bond with cyano ligand, differs significantly from 1·10H2O where the HCF4– core and BPA4+ are bridged via H-bonded water molecules. The in-situ temperature-dependent ESR study showed that structural changes are accompanied by the intramolecular HCF4–-to-BPA4+ one-electron transfer, resulting in the formation of a radical BPA4• species and a small quantity of BPA4•/HCF3– pair within complex 1. To supplement these experimental results and to provide further insight into the observed hydrochromism, comparative DFT calculations of the density of states using the crystallographic data of both CTCs, 1·10H2O and 1 is performed. The observed full extent of one dehydration/rehydration cycle for this redox-active CTC enables the redox-activity to be switched-on within anhydrous complex 1 and switched-off within hydrated complex 1·10H2O.