The hydrothermal synthesis and the ab initio structure determination from powder data of K 2 (V V O 2 ) 4 (V I V O){O 3 P-CH 2 -PO 3 } 2 -xH 2 O (x=4) or MIL-33 are presented. MIL-33 crystallizes in the monoclinic system [space group C2/m (No. 12)] with lattice parameters a=4.8716(4) 9, b=14.614(1) 9, c=15.825(1) 9, β=94.06(1) 9, V=1123.8(2) 9 3 , Z=2. MIL-33 is a mixed valence V I V /V V =1/4 vanadodiphosphonate. Its layered structure shows some potassium cations located both between the hybrid layers and in the eight-membered windows of the latter.