A hydrothermal reaction of a mixture of Eu 2 O 3 , V 2 O 5 , 4,4 0 -bpy, phen, HCl and ethylenediamine in methanol solution resulted in the unexpected complex, [V 6 O 12 (phen) 4 (CH 3 O) 4 ] . 4H 2 O, which was then characterized by single-crystal X-ray diffraction. This complex crystallizes in the triclinic system with the space group P-1 and cell parameters a 5 9.778(4) Å , b 5 11.620(5) Å , c 5 13.271(5) Å , a 5 73.433(8)8 8 8 8 8, b 5 77.068 (8)8 8 8 8 8, g 5 87.509(8)8 8 8 8 8, and Z52 with R 1 5 0.0726. Interestingly, a cyclic water tetramer is located in the packing diagrams of the title complex. The four water molecules linked by hydrogen bonds generate a two-dimensional (2D) box structure, resulting in the formation of a square-prism like water tetramer, which has not been experimentally observed and theoretically predicted so far.