Solvothermal reactions with different solvents produced two iron trimesates [Fe-2(H2O)(2)(BTC)(4/3)]Cl center dot 4.5(DMF) (1) and [Fe4Cl(BTC)(8/3)]Cl-2 center dot H2O center dot 2.5(DEF) (2) (BTC = 1,3,5-benzenetricarboxylate, DMF = N,N'-dimethylformamide, DEF = N,N'-diethylformamide). The framework of 1 is a (3,4)-connected net constructed from mixed-valence paddlewheel Fe-2(II, III) units and BTC linkers, while the framework of 2 is a (3,8)-connected net built from mixed-valence square-planar Fe-4(III, III, III, II) Units and BTC linkers. The large volume inside the framework of 1 (or 2) is occupied by disordered Cl- anions and guest DMF (or DEF) molecules. The mixed-valence character of the frameworks of 1 and 2 was confirmed by Mossbauer spectroscopy studies. The active electronic property of iron cations may be the origin of the variability of the iron-organic frameworks, which are readily affected by some synthetic factors, such as solvents. Magnetic studies reveal that there are antiferromagnetic exchange interactions among the Fe atoms in 1 and 2. Ion-exchange studies for 1 show that the Cl- anions inside the framework of 1 can be exchanged by CNS- anions.