1-Hydroxy-3-methoxy xanthone forms a complex ThL₄ in aqueous alcoholic solution at pH 2·6–4·0 and it can be precipitated with water, dried and ignited to yield thorium dioxide. This provides a satisfactory method of estimation of the metal in the presence of a large excess of cerite earths and ceric salts since these are not precipitated by this reagent and also in the presence of uranium which gets precipitated only at higher pH. The method can be used for separation also.