Nitroprussides of divalent transition metals form a family of microporous molecular materials. Their properties in this sense depend on the transition metal cation involved and also on the preparative method, which determine their crystal structures. The stable phases of this family of materials belong to one of the following crystal structures: orthorhombic (Pnma) (Mn2+, Fe2+, Cu2+, Zn2+, and Cd2+), cubic (Fm3m) (CO2+ and Ni2+) and orthorhombic (Amm2) (Cu2+). These materials are stable up to above 160degreesC, while their dehydration takes place around 100degreesC. On dehydration, Amm2 copper complex changes into a tetragonal (14mm) phase. The microporous nature of these materials is discussed according to their crystal structure and correlating structural and adsorption data. The accessibility to the pore system was evaluated through adsorption of H2O, CO2, and N-2. Pores of both orthorhombic and cubic structures are accessible to H2O and CO2 in experiments carried out at 23 and 0degreesC, respectively; however, they are inaccessible to N-2 at -196degreesC. This behavior is discussed as related to the large polarizing power of the nitrosyl (NO) ligand which distorts the local environment of the iron atom and reduces the effective window cross section. The small pores of tetragonal copper nitroprusside were inaccessible to the adsorbates used.