Two new nickel metavanadate hydrates, Ni(VO 3 ) 2 ' 4H 2 O and -Ni(VO 3 ) 2 ' 2H 2 O, have been synthesized using a 99chimie douce:: process; their crystal structures have been solved from single-crystal X-ray di4raction data. The tetrahydrate Ni(VO 3 ) 2 ' 4H 2 O crystallizes in the monoclinic space group C2/c with a unit cell of dimensions a ؍ 12.969(5) A > , b ؍ 9.817(3) A > , c ؍ 6.880(2) A > , ؍ 112.14(3)3; Z ؍ 4; the structure was determined from 1055 independent re6ections and re5ned until R ؍ 0.024 and R w ؍ 0.034. An orthorhombic cell has been found for the dihydrate -Ni(VO 3 ) 2 ' 2H 2 O: a ؍ 5.530(1) A > , b ؍ 10.621(10) A > , c ؍ 11.756(5) A > , Z ؍ 4, space group Pnma; the structure was determined from 1201 independent re6ections until R ؍ 0.017 and R w ؍ 0.018. The structures of the two materials mainly di4er in the vanadium environment in the (VO 3 ) n؊ n metavanadate chains. They are constituted of edge-linked VO 5 square pyramids in the tetrahydrate and from vertex-linked VO 4 tetrahedra in the dihydrate. As the hydrogen atoms were localized in these two compounds, all the hydrogen bonds existing in these structures were clearly evidenced.