MIL-21 or [Fe III 5−x V III x (H 2 PO 4 ) 4 (HPO 4 ) 4 F 4 (H 2 O) 2 , 4(H 2+y N–(CH 2 ) 2 –NH 2+y )] was prepared hydrothermally (453 K, autogenous pressure, 3 days) in the presence of ethylenediamine. Its structure was determined by single-crystal X-ray diffraction. MIL-21 is tetragonal (space group P-42 1 c, No. 114, with cell parameters a=17.2023(2) Å, c=13.3086(2) Å, V=3938.27(9) Å 3 , Z=4. Its three-dimensional network can be described from the connection via phosphate tetrahedra of infinite corner-sharing cis–trans octahedral chains of metallic atoms in a random distribution and isolated FeO 4 (H 2 O) 2 octahedra. The (Fe 3+ , V 3+ ) infinite chains belong to the KTP type. Mössbauer and structure refinements demonstrate this segregation. This structure exhibits small tunnels and cages in which the ethylenediamine molecules are located. Moreover, MIL-21 shows a ferrimagnetic behavior below 13 K.