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