Two Fe^II-based molybdenum polycyanides, Fe₂[Mo^III(CN)₇]·8H₂O and [Fe₂(H₂O)₄][Mo^IV(CN)₈]·4H₂O, have been synthesized. The compound [Fe₂(H₂O)₄][Mo^IV(CN)₈]·4H₂O crystallizes in the tetragonal system, space group I422. There are two molybdenum sites in a distorted square antiprism arrangement, each site being surrounded by eight CN–Fe linkages. The distorted octahedral iron site is formed by four NC–Mo linkages and two water molecules in apical positions. The structure is three-dimensional and highly symmetrical. The magnetic characteristics of these two compounds were compared. Fe₂[Mo^III(CN)₇]·8H₂O orders below 65 K as a ferrimagnet, but [Fe₂(H₂O)₄][Mo^IV(CN)₈]·4H₂O shows no evidence of long-range magnetic order, obviously due to the contribution of the diamagnetic Mo^IV which suppresses the propagation of magnetic interaction between adjacent iron(II) ions through CN bridges. The results are discussed in the light of the single-crystal structural features of [Fe₂(H₂O)₄][Mo^IV(CN)₈]·4H₂O.