The dehydration of CoSeO4.6H(2)O, CoSeO4.5H(2)O CoSeO4.4H(2)O and CoSeO4.H2O has been studied by TG and DTA and DSC. The dehydration of CoSeO4.6H(2)O occurs in steps and intermediate hydrates CoSeO4.5H(2)O, CoSeO4.4H(2)O, CoSeO4.2H(2)O and CoSeO4.H2O are formed. The enthalpies of dehydration of the observed dehydration processes have been determined. The enthalpies of formation of CoSeO4.6H(2)O; CoSeO4.5H(2)O; CoSeO4.4H(2)O and CoSeO4.H2O (amorphous and crystalline) as well as the enthalpy of transformation of amorphous CoSeO4.H2O into crystalline CoSeO4.H2O have been calculated from DSC data. The lattice parameters of CoSeO4.H2O have been calculated. It crystallizes in a monoclinic system with lattice constants : a=6.001(2) Angstrom; b=13.836(3) Angstrom; c=8.059(3) Angstrom; beta=90.91(4)degrees; V=669.2(3) Angstrom; SG P2(1)/n.