Dehydration processes of the hydrous oxides of thorium, lanthanum and yttrium obtained by addition of ammonium hydroxide to the chloride solutions were examined by thermogravimetric, differential thermal and x-ray diffraction analyses. It was observed that the hydrous oxides of thorium, cerium and yttrium were amorphous and belong to the group of aquated oxides, and hydrous oxide of lanthanum was lanthanum tri-hydroxide. The tri-hydroxide is dehydrated according to the process, La(OH)₃, 270℃ LaOOH 680℃ La₂O₃.