The thermal dehydration of NiSeO4 . 6H(2)O, NiSeO4 . 4H(2)O and NiSeO4 . H2O has been studied by TG, DTA and DSC. The dehydration of NiSeO4 . 6H(2)O depends on both the heating rate and the sample mass. The following intermediate hydrates are produced : NiSeO4 . 5H(2)O; NiSeO4 . 4H(2)O; NiSeO4 . 2H(2)O; NiSeO4 . H2O and NiSeO4 . 0.5H(2)O. The enthalpies of dehydration of these stages have been determined. The enthalpies of formation of NiSeO4 . 6H(2)O; NiSeO4 . 4H(2)O and NiSeO4 . H2O (amorphous and crystalline) as well as the enthalpy of transformation of amorphous NiSeO4 . H2O into crystalline NiSeO4 . H2O have been calculated from DSC data. The lattice parameters of NiSeO4 . 4H(2)O and NiSeO4 . 2H(2)O have been calculated. NiSeO4 . 4H(2)O crystallizes in the monoclinic system with lattice parameters : a = 5.959(3) Angstrom; b = 13.771(8) Angstrom; c = 8.023(5) Angstrom; beta = 90.66(6); V = 658.3(5) Angstrom(3), SG P2(1)/n. NiSeO4 . 2H(2)O forms orthorhombic crystals with lattice parameters : a = 10.351(4) Angstrom; b = 10.219(4) Angstrom; c = 9.017(5) Angstrom; V = 953.9(6) Angstrom(3).