The new 3D Hofmann-type coordination polymer [Fe(dpyu){Pt(CN)(4)}]center dot 9H(2)O [dpyu = 1,3-di(pyridin-4-yl)urea] exhibits reversible interchange between two- and one-step spin-crossover behavior, associated with desorption/resorption of lattice water molecules. Solvent water removal also induces an increase of the spin-transition temperature, indicating strong lattice cooperativity, observed for the first time in a 3D Hofmann-type coordination polymer.