The anthraquinone emodin (9) was rapidly and efficiently converted into catenarin (13) by treatment with a mixture of oleum and boric acid at room temperature. Chrysophanol (15) similarly gave islandicin (16), while related anthraquinones were hydroxylated in essentially the same way, though none as rapidly as emodin. The specificity of such processes is synthetically useful in generally involving substitution of the less highly hydroxylated ring. This is in contrast to hydroxylation with persulfate in sulfuric acid which, for emodin, occurred chiefly in the more highly hydroxylated ring, as did sulfonation.