The crystal structure of the new oxide Pb 2 V 5 O 12 was determined from X-ray single crystal and neutron powder diffraction data. The oxide is monoclinic, space group P2/c with a = 8.408(2), b = 5.017(1), c = 11.940(2) A Ê , b = 98.42(3)°, Z = 2. The crystal structure of Pb 2 V 5 O 12 consists of isolated layers formed by edge and corner sharing VO 5 square pyramids with short vanadyl bonds directed above and below the layers. Six pyramids linked by corner form so-called ªtiling plaquettesº. Neighboring plaquettes are linked by edge sharing. The Pb cations are located in the interstitial positions between layers. The structure of Pb 2 V 5 O 12 is closely related to the MV IV n O 2n+1 homologous series known for Ca, Sr and Cd. Electron diffraction and high resolution electrom microscopy study confirm the crystal structure data and revealed a phase transition induced by irradia-tion resulting in the formation of a disordered orthorhombic phase (a » 3.75, b » 3.79, c » 5.03 A Ê ) which corresponds to a sublattice of the plaquette-built structure. The analyses of related structures allowed to suggest a more general formula for this homologous series: M m V m+n O 3m+2n . The structures of such compounds may be described by shear planes formed by a shift of the VO 2 units over a 1/2* [110] translation starting from the parent ªVO 2 º or ªMVO 3 º structures. Magnetic susceptibility measurements of Pb 2 V 5 O 12 indicate low dimensional behavior and a strong antiferromagnetic exchange within the plaquettes with J of the order of 190 K. Anomalies observed in the susceptibility and the specific heat indicate magnetic order at T N = 26 K.