The influence of lattice water in the magnetic properties of spin-crossover [Fe(bpp)(2)]X-2-nH(2)O salts [bpp = 2,6-bis(pyrazol-3-yl)pyridine] is well-documented. In most cases, it stabilizes the low-spin state compared to the anhydrous compound. In other cases, it is rather the contrary. Unraveling this mystery implies the study of the microscopic changes that accompany the loss of water. This might be difficult from an experimental point of view. Our strategy is to focus on some salts that undergo a nonreversible dehydration-hydration process without loss of crystallinity. By comparison of the structural and magnetic properties of original and rehydrated samples, several rules concerning the role of water at the microscopic level can be deduced. This paper reports on the crystal structure, thermal studies, and magnetic properties of [Fe(bpp)(2)][Cr(bpy)(ox)(2)](2)center dot 2H(2)O (1), [Fe(bPP)(2)][Cr(phen)(ox)(2)](2)center dot 5H(2)O center dot 0.5MeOH (2), and [Fe(bpp)(2)][Cr(phen)(ox)(2)](2)center dot 5.5H(2)O center dot 2.5MeOH (3). Salt 1 contains both high-spin (HS) and low-spin (LS) Fe2+ cations in a 1:1 ratio. Dehydration yields the anhydrous spin-crossover compound with T-1/2 down arrow = 353 K and T-1/2 up arrow = 369 K. Rehydration affords the dihydrate [Fe(bPP)(2)][Cr(bpy)(ox)(2)](2)center dot 2H(2)O (1r) with 100% HS Fe2+ sites. Salt 2 also contains both HS and LS Fe2+ cations in a 1:1 ratio. Dehydration yields the anhydrous spin-crossover compound with T-1/2 down arrow = 343 K and T-1/2 up arrow = 348 K. Rehydration affords [Fe(bPP)(2)][Cr(phen)(ox)(2)](2)center dot 0.5H(2)O (2r) with 72% Fe2+ sites in the LS configuration. The structural, magnetic, and thermal properties of these rehydrated compounds 1r and 2r are also discussed. Finally, 1 has been dehydrated and resolvated with MeOH to give [Fe(bPP)(2)][Cr(bpy)(ox)(2)](2)center dot MeOH (1s) with 33% HS Fe2+ sites. The influence of the guest solvent in the Fe2+ spin state can anticipate the future applications of these compounds in solvent sensing.